Also run activate_xcode on macos_arm64 platform (#1159)

Fixing https://buildkite.com/bazel-trusted/publish-bazel-binaries/builds/9855#e62d31bb-f02f-45c0-8dc5-8de75f786970
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 1cb5020..fe12cd6 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1027,7 +1027,7 @@
     tmpdir = tempfile.mkdtemp()
     sc_process = None
     try:
-        if platform == "macos":
+        if platform == "macos" or platform == "macos_arm64":
             activate_xcode(task_config)
 
         # If the CI worker runs Bazelisk, we need to forward all required env variables to the test.