chromium.run_mb: set cwd to checkout

scripts/slave/recipes/v8.expected/full_tryserver_v8_v8_mac_rel_ng.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.v8",
       "--builder",
       "v8_mac_rel_ng",
@@ skipping 164 matching lines @@
       "tryserver.v8",
       "-b",
       "v8_mac_rel_ng",
       "--config-file",
       "[SLAVE_BUILD]/v8/infra/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--gyp-script=gypfiles/gyp_v8",
       "//out/Release"
     ],
+    "cwd": "[SLAVE_BUILD]/v8",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@stdout@some line@@@",
       "@@@STEP_LOG_LINE@stdout@GYP_DEFINES='target_arch=x64 cool_flag=a=1'@@@",
       "@@@STEP_LOG_LINE@stdout@moar@@@",
       "@@@STEP_LOG_END@stdout@@@"
@@ skipping 40 matching lines @@
       "-m",
       "tryserver.v8",
       "-b",
       "v8_mac_rel_ng",
       "--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 140 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@v8_mac_rel_ng_triggered@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]