[infra] Use mac_use_goma_rbe=true on dart-sdk-mac

This uses the build feature added by:
https://dart-review.googlesource.com/c/sdk/+/139782

Bug:b/151697301
Cq-Include-Trybots: dart/try:dart-sdk-mac-try
Change-Id: Ia8e748c1b8f0bc8836d94b110e686c41d3b69094
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153982
Auto-Submit: Alexander Thomas <[email protected]>
Commit-Queue: William Hesse <[email protected]>
Reviewed-by: William Hesse <[email protected]>

Author: athomas

tools/bots/test_matrix.json

@@ -2714,7 +2714,10 @@
             "--arch=x64",
             "--mode=release",
             "create_sdk"
-          ]
+          ],
+          "environment": {
+            "DART_GN_ARGS": "mac_use_goma_rbe=true"
+          }
         },
         {
           "name": "upload sdk",