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

scripts/slave/recipes/v8.expected/full_client_v8_V8_Fuzzer_fuzz_archive.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:Fuzz",
+      "stepname:Fuzz on Ubuntu-12.04",
       "--idempotent",
       "[dummy hash for jsfunfuzz]",
       "--",
       "out/Debug/d8",
       "${ISOLATED_OUTDIR}/fuzz-results-54321.tar.bz2"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] Fuzz",
+    "name": "[trigger] Fuzz on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"Fuzz/Ubuntu-12.04/[dummy has/V8 Fuzzer/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"Fuzz/Ubuntu-12.04/[dummy has/V8 Fuzzer/571\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
@@ skipping 14 matching lines @@
       "--print-status-updates",
       "--shards",
       "1",
       "Fuzz/Ubuntu-12.04/[dummy has/V8 Fuzzer/571",
       "--task-summary-json",
       "/path/to/tmp/json",
       "--task-output-dir",
       "[TMP_BASE]/swarming_output_tmp_1"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "Fuzz",
+    "name": "Fuzz on Ubuntu-12.04",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "cmd": [
       "python",
@@ skipping 11 matching lines @@
     "~followup_annotations": [
       "@@@STEP_LINK@gsutil.upload@https://storage.cloud.google.com/chromium-v8/fuzzer-archives/fuzz-results-54321.tar.bz2@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 tests failed: ['jsfunfuzz']",
     "status_code": 1
   }
 ]