chromium_tests: apply chromium.get_env() when running tests

scripts/slave/recipes/chromium.expected/one_failure_keeps_going_dynamic_tests.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.linux",
       "--builder",
       "Linux Tests",
@@ skipping 295 matching lines @@
       "--test-launcher-summary-output",
       "/path/to/tmp/json",
       "--generate-json-file",
       "-o",
       "gtest-results/base_unittests",
       "--builder-name=Linux Tests",
       "--slave-name=TestSlavename",
       "--build-number=571",
       "base_unittests"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "base_unittests",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
@@ skipping 15 matching lines @@
       "--generate-json-file",
       "-o",
       "gtest-results/browser_tests",
       "--builder-name=Linux Tests",
       "--slave-name=TestSlavename",
       "--build-number=571",
       "browser_tests",
       "--test-launcher-shard-index=0",
       "--test-launcher-total-shards=2"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "browser_tests"
   },
   {
     "name": "$result",
     "reason": "1 tests failed: ['base_unittests']",
     "status_code": 1
   }
 ]