chromium.run_mb: set cwd to checkout

scripts/slave/recipes/v8.expected/full_client_v8_V8_Win64.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 167 matching lines @@
       "client.v8",
       "-b",
       "V8 Win64",
       "--config-file",
       "[SLAVE_BUILD]\\v8\\infra\\mb\\mb_config.pyl",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "--gyp-script=gypfiles\\gyp_v8",
       "//out/Release_x64"
     ],
+    "cwd": "[SLAVE_BUILD]\\v8",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json",
       "GYP_MSVS_VERSION": "2013"
     },
     "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@@@",
@@ skipping 41 matching lines @@
       "-m",
       "client.v8",
       "-b",
       "V8 Win64",
       "--config-file",
       "[mb config]",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "[SLAVE_BUILD]\\v8\\out\\gn"
     ],
+    "cwd": "[SLAVE_BUILD]\\v8",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json",
       "GYP_MSVS_VERSION": "2013"
     },
     "name": "generate_build_files with gn (fyi)"
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 748 matching lines @@
       "@@@STEP_LOG_LINE@durations@Duration: 00:00:101@@@",
       "@@@STEP_LOG_END@durations@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]