Prepare brew cache before upload.

.cirrus.yml

@@ -75,6 +75,12 @@ macos_task_template: &MACOS_TASK_TEMPLATE
     - make -j$(nproc)
   deploy_script:
     - $CIRRUS_WORKING_DIR/scripts/cirrus-deploy.sh
+  before_cache_script:
+    - brew cleanup
+    # Credit https://discourse.brew.sh/t/best-practice-for-homebrew-on-travis-brew-update-is-5min-to-build-time/5215/9
+    # Cache only .git files under "/usr/local/Homebrew" so "brew update" does not take 5min every build
+    - find  /usr/local/Homebrew \! -regex ".+\.git.+" -delete
+    - find  $HOME/Library/Caches/Homebrew \! -regex ".+\.git.+" -delete
   env:
     COMPILER: clang