Enable hermetic builds for FYI mac builders.

scripts/slave/recipes/chromium.expected/full_chromium_fyi_ClangToTMacASan.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.fyi",
       "--builder",
       "ClangToTMacASan",
@@ skipping 107 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_DEFINES": "asan=1 clang=1 clang_use_chrome_plugins=0 component=static_library fastbuild=0 target_arch=x64 test_isolation_mode=prepare",
       "LLVM_FORCE_HEAD_REVISION": "YES",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
@@ skipping 94 matching lines @@
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--output-json",
       "/path/to/tmp/json"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "LLVM_FORCE_HEAD_REVISION": "YES"
     },
     "name": "clang_revision",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"clang_revision\": \"123456-7\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
@@ skipping 21 matching lines @@
       "--build-data-dir",
       "[ROOT]/build_data_dir",
       "--compiler",
       "clang",
       "--target-output-dir",
       "[SLAVE_BUILD]/src/out/Release",
       "--",
       "all"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "LLVM_FORCE_HEAD_REVISION": "YES"
     },
     "name": "compile"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::isolate]/resources/remove_build_metadata.py",
@@ skipping 108 matching lines @@
       "mac"
     ],
     "name": "sizes"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]