ios: fix cwd for git diff

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

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 295 matching lines @@
     },
     "name": "generate_build_files (with patch)"
   },
   {
     "cmd": [
       "git",
       "diff",
       "--cached",
       "--name-only"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[BUILDER_CACHE]/ios_simulator_gn/src",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@files@foo.cc@@@",
       "@@@STEP_LOG_END@files@@@"
     ]
   },
   {
     "cmd": [
       "python",
@@ skipping 67 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@This test was skipped because it was not affected.@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]