ci: cache Homebrew tool downloads

.github/workflows/shadowsocksx-ng2-ci.yml

@@ -26,9 +26,48 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: Prepare Homebrew download cache
+        id: homebrew-cache-key
+        run: |
+          brew info --json=v2 age go-task xcodegen swiftlint > "$RUNNER_TEMP/homebrew-formulae.json"
+          formula_hash=$(python3 - "$RUNNER_TEMP/homebrew-formulae.json" <<'PY'
+          import hashlib
+          import json
+          import sys
+
+          with open(sys.argv[1]) as source:
+              formulae = json.load(source)["formulae"]
+          versions = sorted(
+              (f["full_name"], f["versions"]["stable"], f["revision"], f["bottle"])
+              for f in formulae
+          )
+          print(hashlib.sha256(json.dumps(versions, sort_keys=True).encode()).hexdigest())
+          PY
+          )
+          echo "path=$(brew --cache)/downloads" >> "$GITHUB_OUTPUT"
+          echo "key=ng2-homebrew-downloads-v1-macos-$(sw_vers -productVersion)-$(uname -m)-$formula_hash" >> "$GITHUB_OUTPUT"
+
+      - name: Restore Homebrew download cache
+        id: homebrew-cache
+        uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ steps.homebrew-cache-key.outputs.path }}
+          key: ${{ steps.homebrew-cache-key.outputs.key }}
+
       - name: Install build tools and test dependencies
+        id: install-build-tools
+        env:
+          HOMEBREW_NO_AUTO_UPDATE: '1'
+          HOMEBREW_NO_INSTALL_CLEANUP: '1'
         run: brew install age go-task xcodegen swiftlint
 
+      - name: Save Homebrew download cache
+        if: steps.install-build-tools.outcome == 'success' && steps.homebrew-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ steps.homebrew-cache-key.outputs.path }}
+          key: ${{ steps.homebrew-cache-key.outputs.key }}
+
       - name: Restore external binary archive cache
         id: external-binary-cache
         uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0