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

scripts/slave/recipes/webrtc/ios.expected/trybot.json

Index: scripts/slave/recipes/webrtc/ios.expected/trybot.json
diff --git a/scripts/slave/recipes/webrtc/ios.expected/trybot.json b/scripts/slave/recipes/webrtc/ios.expected/trybot.json
index 795dad56db6b2fec6b65abd6a499d3fb8d59dfab..7585bfd5180007b739da87fa3237cb77bad2c17c 100644
--- a/scripts/slave/recipes/webrtc/ios.expected/trybot.json
+++ b/scripts/slave/recipes/webrtc/ios.expected/trybot.json
@@ -240,6 +240,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/ios/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
@@ -256,6 +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",
@@ -305,6 +307,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/ios/src/out/Release-iphoneos",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",