ios: Allow bot configs to set extra env variables

scripts/slave/recipes/ios/unified_builder_tester.expected/no_tests.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 96 matching lines @@
       "@@@STEP_LOG_LINE@json.output@  \"tests\": [], @@@",
       "@@@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\": \"Release\", \n  \"sdk\": \"iphoneos8.0\", \n  \"tests\": [], \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\": \"Release\", \n  \"env\": {}, \n  \"sdk\": \"iphoneos8.0\", \n  \"tests\": [], \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 28 matching lines @@
   {
     "cmd": [
       "ninja",
       "-C",
       "[SLAVE_BUILD]/src/out/Release-iphoneos"
     ],
     "cwd": "[SLAVE_BUILD]/src/out/Release-iphoneos",
     "name": "compile"
   }
 ]