chromium.run_mb: set cwd to checkout

scripts/slave/recipes/chromium.expected/full_chromium_fyi_Win_10_Fast_Ring.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 250 matching lines @@
       "-m",
       "chromium.fyi",
       "-b",
       "Win 10 Fast Ring",
       "--config-file",
       "[BUILDER_CACHE]\\Win_10_Fast_Ring\\src\\tools\\mb\\mb_config.pyl",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "//out/Release_x64"
     ],
+    "cwd": "[BUILDER_CACHE]\\Win_10_Fast_Ring\\src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\tools\\runit.py",
@@ skipping 34 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]