chromium.run_mb: set cwd to checkout

scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_linux_release.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.dart.fyi",
       "--builder",
       "v8-linux-release",
@@ skipping 168 matching lines @@
       "-m",
       "client.dart.fyi",
       "-b",
       "v8-linux-release",
       "--config-file",
       "[mb config]",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "[SLAVE_BUILD]/v8/out/gn"
     ],
+    "cwd": "[SLAVE_BUILD]/v8",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files with gn (fyi)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/v8/tools/gyp_flag_compare.py",
@@ skipping 75 matching lines @@
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"branch\": \"4.2\", \"buildername\": \"v8-linux-release\", \"buildnumber\": 571, \"mastername\": \"client.dart.fyi\", \"parent_buildername\": null, \"recipe\": \"v8\", \"revision\": \"20123\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]