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

scripts/slave/recipes/v8.expected/full_tryserver_v8_v8_linux_nosnap_dbg.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.v8",
       "--builder",
       "v8_linux_nosnap_dbg",
@@ skipping 223 matching lines @@
       "project:v8",
       "--tag",
       "purpose:CQ",
       "--tag",
       "purpose:pre-commit",
       "--tag",
       "rietveld:https://codereview.chromium.org/12853011/#ps1",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:Check",
+      "stepname:Check on Ubuntu-12.04",
       "--idempotent",
       "--user",
       "commit-bot@chromium.org",
       "[dummy hash for bot_default]",
       "--",
       "--progress=verbose",
       "--mode",
       "Debug",
       "--arch",
       "ia32",
       "--outdir",
       "out",
       "--buildbot",
       "--timeout=200",
       "--random-seed=-976563914",
       "bot_default",
       "--exhaustive-variants",
       "--dcheck-always-on",
       "--rerun-failures-count=2",
       "--swarming",
       "--json-test-results",
       "${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] Check",
+    "name": "[trigger] Check on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"Check/Ubuntu-12.04/[dummy has/v8_linux_nosnap_dbg/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"Check/Ubuntu-12.04/[dummy has/v8_linux_nosnap_dbg/571:3:0\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"Check/Ubuntu-12.04/[dummy has/v8_linux_nosnap_dbg/571:3:1\": {@@@",
@@ skipping 54 matching lines @@
       "@@@STEP_LOG_LINE@durations@Duration: 00:00:101@@@",
       "@@@STEP_LOG_END@durations@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]