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

scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json

Index: scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json
diff --git a/scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json b/scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json
index aee3814b6c4caf5e1ed05d1c567df8e8b5f98587..05c297ab31a8433d09f04659a4cddb73f1c79180 100644
--- a/scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json
+++ b/scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json
@@ -118,6 +118,7 @@
     ],
     "cwd": "[SLAVE_BUILD]/v8",
     "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": "clang=1 gomadir='[CACHE]/cipd/goma' target_arch=ia32 use_goma=1 v8_enable_disassembler=1 v8_target_arch=ia32",
@@ -156,6 +157,7 @@
     ],
     "cwd": "[SLAVE_BUILD]/v8",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files",
@@ -217,6 +219,7 @@
     ],
     "cwd": "[SLAVE_BUILD]/v8",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files with gn (fyi)"
@@ -254,6 +257,7 @@
       "/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",
@@ -301,6 +305,7 @@
       "--"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",