[build] Set mac_use_goma_rbe to true for goma builds on mac

Without this, local builds on macOS fail to use the Goma on RBE backend.

See also: https://dart-review.googlesource.com/c/sdk/+/139782

Change-Id: Ic1caba904bcb486c638aef9462bca3fda65d8a6a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152147
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>

Author: athomas

tools/gn.py

@@ -265,6 +265,9 @@ def ToGnArgs(args, mode, arch, target_os, sanitizer, verify_sdk_hash):
         gn_args['use_goma'] = False
         gn_args['goma_dir'] = None
 
+    if gn_args['target_os'] == 'mac' and gn_args['use_goma']:
+        gn_args['mac_use_goma_rbe'] = True
+
     # Code coverage requires -O0 to be set.
     if enable_code_coverage:
         gn_args['dart_debug_optimization_level'] = 0