ios: Allow bot configs to set extra env variables

scripts/slave/recipes/ios/try.expected/basic.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 106 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  ], @@@",
       "@@@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\": \"ninja\", \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  \"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\": \"ninja\", \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  \"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 93 matching lines @@
   },
   {
     "cmd": [],
     "cwd": "[SLAVE_BUILD]",
     "name": "[skipped] fake tests (fake device iOS 8.1)",
     "~followup_annotations": [
       "@@@STEP_TEXT@This test was skipped because it was not affected.@@@"
     ]
   }
 ]