Recipes: make cwd default to slave_build

scripts/slave/recipes/infra/sphinx_doc_updater.expected/basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "fake",
       "--builder",
       "fake",
       "--slave",
       "fake",
       "--spec",
       "cache_dir = '[ROOT]/git_cache'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'infra', 'url': 'https://chromium.googlesource.com/infra/infra.git'}]",
       "--root",
       "infra",
       "--revision_mapping_file",
       "{\"infra\": \"got_revision\"}",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "infra@HEAD",
       "--force"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_root\": \"infra\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision\": \"5e03a58dc0850e5137a8ebbce29610670c756aed\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision_cp\": \"refs/heads/master@{#197293}\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"infra\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_revision@\"5e03a58dc0850e5137a8ebbce29610670c756aed\"@@@",
       "@@@SET_BUILD_PROPERTY@got_revision_cp@\"refs/heads/master@{#197293}\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/infra/docgen.py",
       "clean"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "Clean documentation"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/infra/docgen.py"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "Generate documentation"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gsutil.py",
       "--force-version",
       "3.25",
       "--",
       "cp",
       "-R",
       "[SLAVE_BUILD]/infra/doc/html",
       "gs://chromium-infra-docs/infra"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gsutil upload",
     "~followup_annotations": [
       "@@@STEP_LINK@Deploy documentation to GCS@https://storage.cloud.google.com/chromium-infra-docs/infra@@@"
     ]
   }
 ]