Download the hermetic xcode toolchain on all bots that target mac.

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

Index: scripts/slave/recipes/ios/try.expected/gn.json
diff --git a/scripts/slave/recipes/ios/try.expected/gn.json b/scripts/slave/recipes/ios/try.expected/gn.json
index d9c9be61b60a6d4bf3b4c9d7dd265d5e15fb9203..7ef4ebbabc80202ba7415a555a15ce78c09fa09a 100644
--- a/scripts/slave/recipes/ios/try.expected/gn.json
+++ b/scripts/slave/recipes/ios/try.expected/gn.json
@@ -300,6 +300,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/ios_simulator_gn/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files (with patch)"
@@ -392,6 +393,7 @@
       "/path/to/tmp/json"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "clang_revision",
@@ -444,6 +446,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/ios_simulator_gn/src/out/Debug-iphonesimulator",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",