chromium_tests: use correct cwd for analyze diff

scripts/slave/recipes/chromium_trybot.expected/swarming_trigger_failure.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/linux_chromium_rel_ng",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 267 matching lines @@
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "git",
       "diff",
       "--cached",
       "--name-only"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[BUILDER_CACHE]/linux_chromium_rel_ng/src",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@files@foo.cc@@@",
       "@@@STEP_LOG_END@files@@@",
       "@@@SET_BUILD_PROPERTY@subproject_tag@\"chromium\"@@@"
     ]
   },
   {
     "cmd": [
@@ skipping 254 matching lines @@
       "@@@SET_BUILD_PROPERTY@failure_hash@\"212d0eb2385b1eaa828053689225258ece065725\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('base_unittests') failed with return_code 1",
     "recipe_result": null,
     "status_code": 1
   }
 ]