Roll recipe dependencies (nontrivial).

scripts/slave/recipes/ios/unified_builder_tester.expected/basic.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 46 matching lines @@
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--output_manifest"
     ],
     "cwd": "[BUILDER_CACHE]/ios",
     "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 266 matching lines @@
       "@@@STEP_LOG_END@fake log@@@",
       "@@@STEP_LINK@fake URL text@fake URL@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]