Roll recipe dependencies (nontrivial).

scripts/slave/recipes/webrtc/auto_roll_webrtc_deps.expected/rolling_activated.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/tools/pycurl.py",
       "https://webrtc-roll-cr-rev-status.appspot.com/status"
     ],
     "name": "check roll status",
     "stdout": "/path/to/tmp/",
@@ skipping 16 matching lines @@
       "{\"src\": \"got_revision\"}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--output_manifest"
     ],
     "env": {
-      "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
+      "PATH": "RECIPE_PACKAGE_REPO[depot_tools]:%(PATH)s"
     },
     "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@  \"fixed_revisions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"src\": \"HEAD\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"manifest\": {@@@",
@@ skipping 58 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "autoroll chromium_revision"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]