Recipes: make cwd default to slave_build

scripts/slave/recipes/gatekeeper.expected/keep_going.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[BUILD]/scripts/slave/gatekeeper_trees.json",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "reading gatekeeper_trees.json",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"blink\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"build-db\": \"blink_build_db.json\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"filter-domain\": \"google.com\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"masters\": [@@@",
       "@@@STEP_LOG_LINE@json.output@      \"https://build.chromium.org/p/chromium.webkit\"@@@",
       "@@@STEP_LOG_LINE@json.output@    ], @@@",
       "@@@STEP_LOG_LINE@json.output@    \"open-tree\": true, @@@",
@@ skipping 13 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/gatekeeper_ng.py",
       "-v",
       "--json",
       "[BUILD]/scripts/slave/gatekeeper.json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gatekeeper: chromium",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 15 matching lines @@
       "--password-file",
       ".blink_status_password",
       "--default-from-email",
       "blink-buildbot@chromium-build.appspotmail.com",
       "--filter-domain",
       "google.com",
       "--status-user",
       "gatekeeper@google.com",
       "https://build.chromium.org/p/chromium.webkit"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gatekeeper: blink"
   }
 ]