recipe_autoroller: always set cwd to workdir

recipes/recipes/recipe_autoroller.expected/repo_data_trivial_cq.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/pycurl.py",
       "https://luci-config.appspot.com/_ah/api/config/v1/projects",
       "--outfile",
       "/path/to/tmp/",
       "--attempts",
@@ skipping 225 matching lines @@
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::gsutil]/resources/gsutil_wrapper.py",
       "--",
       "RECIPE_PACKAGE_REPO[depot_tools]/gsutil.py",
       "----",
       "cat",
       "gs://recipe-roller-cl-uploads/repo_metadata/aHR0cHM6Ly9yZXBvLnJlcG8vYnVpbGQ="
     ],
+    "cwd": "[TMP_BASE]/roll_build_tmp_2",
     "name": "build.gsutil repo_state",
     "stderr": "/path/to/tmp/",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@",
       "@@@STEP_TEXT@ (trivial)@@@",
       "@@@STEP_LINK@Issue 123456789@https://codereview.chromium.org/123456789@@@"
     ]
   },
   {
@@ skipping 34 matching lines @@
       "[TMP_BASE]/recipes_tmp_1"
     ],
     "name": "rmtree [TMP_BASE]/recipes_tmp_1"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]