Revert "stop testing Gerrit on macOS; It hardcodes python2 which is generally not available on macOS."

This reverts commit 7895accf5b2c68c5f20a5393ab70f741b4845197.
diff --git a/buildkite/pipelines/gerrit-postsubmit.json b/buildkite/pipelines/gerrit-postsubmit.json
index 611a936..10bfb9e 100644
--- a/buildkite/pipelines/gerrit-postsubmit.json
+++ b/buildkite/pipelines/gerrit-postsubmit.json
@@ -9,6 +9,11 @@
       "build_targets": ["//:release"],
       "test_flags": ["--test_tag_filters=-slow"],
       "test_targets": ["..."]
+    },
+    "macos": {
+      "build_targets": ["//:release"],
+      "test_flags": ["--test_tag_filters=-slow"],
+      "test_targets": ["..."]
     }
   }
 }