swarming: always append task's OS dimension to step name

scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___nosnap___debug.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::git]/resources/git_setup.py",
       "--path",
       "[SLAVE_BUILD]/swarming.client",
       "--url",
       "https://chromium.googlesource.com/external/swarming.client.git"
@@ skipping 133 matching lines @@
       "os:Ubuntu-12.04",
       "--tag",
       "project:v8",
       "--tag",
       "purpose:CI",
       "--tag",
       "purpose:post-commit",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:Check",
+      "stepname:Check on Ubuntu-12.04",
       "--idempotent",
       "[dummy hash for bot_default]",
       "--",
       "--progress=verbose",
       "--mode",
       "Debug",
       "--arch",
       "ia32",
       "--outdir",
       "out",
       "--buildbot",
       "--timeout=200",
       "--random-seed=-1847850043",
       "bot_default",
       "--exhaustive-variants",
       "--no-snap",
       "--rerun-failures-count=2",
       "--swarming",
       "--json-test-results",
       "${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] Check",
+    "name": "[trigger] Check on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"Check/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"Check/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571:3:0\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"Check/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571:3:1\": {@@@",
@@ skipping 64 matching lines @@
       "os:Ubuntu-12.04",
       "--tag",
       "project:v8",
       "--tag",
       "purpose:CI",
       "--tag",
       "purpose:post-commit",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:Mozilla",
+      "stepname:Mozilla on Ubuntu-12.04",
       "--idempotent",
       "[dummy hash for mozilla]",
       "--",
       "--progress=verbose",
       "--mode",
       "Debug",
       "--arch",
       "ia32",
       "--outdir",
       "out",
       "--buildbot",
       "--timeout=200",
       "--random-seed=-1847850043",
       "mozilla",
       "--exhaustive-variants",
       "--no-snap",
       "--rerun-failures-count=2",
       "--swarming",
       "--json-test-results",
       "${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] Mozilla",
+    "name": "[trigger] Mozilla on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"Mozilla/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"Mozilla/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
@@ skipping 52 matching lines @@
       "os:Ubuntu-12.04",
       "--tag",
       "project:v8",
       "--tag",
       "purpose:CI",
       "--tag",
       "purpose:post-commit",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:SimdJs - all",
+      "stepname:SimdJs - all on Ubuntu-12.04",
       "--idempotent",
       "[dummy hash for simdjs]",
       "--",
       "--progress=verbose",
       "--mode",
       "Debug",
       "--arch",
       "ia32",
       "--outdir",
       "out",
       "--buildbot",
       "--timeout=200",
       "--random-seed=-1847850043",
       "simdjs",
       "--download-data",
       "--exhaustive-variants",
       "--no-snap",
       "--rerun-failures-count=2",
       "--swarming",
       "--json-test-results",
       "${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] SimdJs - all",
+    "name": "[trigger] SimdJs - all on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"SimdJs - all/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"SimdJs - all/Ubuntu-12.04/[dummy has/V8 Linux - nosnap - debug/571\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
@@ skipping 124 matching lines @@
       "@@@STEP_LOG_LINE@durations@Duration: 00:00:101@@@",
       "@@@STEP_LOG_END@durations@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]