Use a shared repocache for all tests on macOS to reduce network traffic (#615)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 9eb0522..395b04d 100644
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1147,9 +1147,17 @@
         "--jobs=" + concurrent_jobs(platform),
         "--announce_rc",
         "--experimental_multi_threaded_digest",
+        "--experimental_repository_cache_hardlinks",
     ]
 
-    if platform != "windows":
+    if platform == "windows":
+        pass
+    elif platform == "macos":
+        flags += [
+            "--sandbox_writable_path=/var/tmp/_bazel_buildkite/cache/repos/v1",
+            "--test_env=REPOSITORY_CACHE=/var/tmp/_bazel_buildkite/cache/repos/v1",
+        ]
+    else:
         flags += ["--sandbox_tmpfs_path=/tmp"]
 
     if bep_file: