bot_update: add depot_tools to PATH so that e.g. apply_issue can be found (reland #1)

recipe_modules/bot_update/example.expected/no_shallow.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "experimental",
       "--builder",
       "Experimental Builder",
       "--slave",
       "somehost",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn.chromium.org/chrome/trunk/src'}]",
       "--root",
       "src",
       "--revision_mapping_file",
       "{\"src\": \"got_cr_revision\"}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--no_shallow"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "PATH": "%(PATH)s:RECIPE_PACKAGE[depot_tools]"
+    },
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": false@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]