chromium.run_mb: set cwd to checkout

scripts/slave/recipes/chromium.expected/full_chromium_win_WinClang64__dbg_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 272 matching lines @@
       "-m",
       "chromium.win",
       "-b",
       "WinClang64 (dbg)",
       "--config-file",
       "[BUILDER_CACHE]\\win_clang\\src\\tools\\mb\\mb_config.pyl",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "//out/Debug_x64"
     ],
+    "cwd": "[BUILDER_CACHE]\\win_clang\\src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
@@ skipping 83 matching lines @@
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]\\\\build_data_dir\", \"buildername\": \"WinClang64 (dbg)\", \"buildnumber\": 571, \"mastername\": \"chromium.win\", \"parent_buildername\": null, \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]