ios: Allow bot configs to set extra env variables

scripts/slave/recipe_modules/ios/example.expected/clobber.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.fake",
       "--builder",
       "ios",
@@ skipping 70 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  \"tests\": [], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"xcode version\": \"6.1.1\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "echo",
       "-e",
-      "{\n  \"GYP_DEFINES\": {\n    \"OS\": \"ios\", \n    \"component\": \"static_library\"\n  }, \n  \"compiler\": \"xcodebuild\", \n  \"configuration\": \"Debug\", \n  \"sdk\": \"iphonesimulator8.1\", \n  \"tests\": [], \n  \"triggered bots\": {}, \n  \"xcode version\": \"6.1.1\"\n}"
+      "{\n  \"GYP_DEFINES\": {\n    \"OS\": \"ios\", \n    \"component\": \"static_library\"\n  }, \n  \"compiler\": \"xcodebuild\", \n  \"configuration\": \"Debug\", \n  \"env\": {}, \n  \"sdk\": \"iphonesimulator8.1\", \n  \"tests\": [], \n  \"triggered bots\": {}, \n  \"xcode version\": \"6.1.1\"\n}"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "finalize build config"
   },
   {
     "cmd": [
       "[BUILD]/scripts/slave/ios/find_xcode.py",
       "--json-file",
       "/path/to/tmp/json",
       "--version",
@@ skipping 32 matching lines @@
       "Debug",
       "-project",
       "[SLAVE_BUILD]/src/build/all.xcodeproj",
       "-sdk",
       "iphonesimulator8.1"
     ],
     "cwd": "[SLAVE_BUILD]/src/xcodebuild",
     "name": "compile"
   }
 ]