ios: Allow bot configs to set extra env variables

scripts/slave/recipes/ios/try.expected/parent.json

 [
   {
     "cmd": [
       "[BUILD]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "host and tools info",
     "~followup_annotations": [
@@ skipping 123 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  \"sdk\": \"iphonesimulator8.0\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"xcode version\": \"fake xcode version\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "echo",
       "-e",
-      "{\n  \"GYP_DEFINES\": {\n    \"OS\": \"ios\", \n    \"component\": \"static_library\", \n    \"fake gyp define 1\": \"fake value 1\", \n    \"fake gyp define 2\": \"fake value 2\"\n  }, \n  \"compiler\": \"xcodebuild\", \n  \"configuration\": \"Debug\", \n  \"sdk\": \"iphonesimulator8.0\", \n  \"tests\": [\n    {\n      \"app\": \"fake tests\", \n      \"device type\": \"fake device\", \n      \"os\": \"8.1\"\n    }\n  ], \n  \"triggered bots\": {}, \n  \"triggered by\": \"parent\", \n  \"xcode version\": \"fake xcode version\"\n}"
+      "{\n  \"GYP_DEFINES\": {\n    \"OS\": \"ios\", \n    \"component\": \"static_library\", \n    \"fake gyp define 1\": \"fake value 1\", \n    \"fake gyp define 2\": \"fake value 2\"\n  }, \n  \"compiler\": \"xcodebuild\", \n  \"configuration\": \"Debug\", \n  \"env\": {}, \n  \"sdk\": \"iphonesimulator8.0\", \n  \"tests\": [\n    {\n      \"app\": \"fake tests\", \n      \"device type\": \"fake device\", \n      \"os\": \"8.1\"\n    }\n  ], \n  \"triggered bots\": {}, \n  \"triggered by\": \"parent\", \n  \"xcode version\": \"fake xcode version\"\n}"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "finalize build config"
   },
   {
     "cmd": [
       "[BUILD]/scripts/slave/ios/find_xcode.py",
       "--json-file",
       "/path/to/tmp/json",
       "--version",
@@ skipping 67 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LOG_LINE@fake log@fake log line 1@@@",
       "@@@STEP_LOG_LINE@fake log@fake log line 2@@@",
       "@@@STEP_LOG_END@fake log@@@",
       "@@@STEP_LINK@fake URL text@fake URL@@@"
     ]
   }
 ]