[CI] Set up CI for Mac M1 (#9699)

tests/buildkite/pipeline-mac-m1.yml

@@ -0,0 +1,8 @@
+steps:
+  - block: ":rocket: Run this test job"
+    if: build.pull_request.id != null || build.branch =~ /^dependabot\//
+  - label: ":macos: Build and Test XGBoost for MacOS M1 with Clang 11"
+    command: "tests/buildkite/test-macos-m1-clang11.sh"
+    key: mac-m1-appleclang11
+    agents:
+      queue: mac-mini-m1