feat: add meta update workflow

This will update metadata json files in the `meta` branch, which are
used for rendering badges for each cask with shields.io.

.github/workflows/update-meta.yml

@@ -0,0 +1,39 @@
+---
+name: Update Meta
+on: [push]
+
+jobs:
+  update-meta:
+    runs-on: macos-10.15
+    if: github.ref == 'refs/heads/main'
+    steps:
+      - name: Checkout tap repository main branch
+        uses: actions/checkout@v2
+        with:
+          path: tap
+      - name: Symlink homebrew tap
+        run: |
+          mkdir -p "$(brew --repo)/Library/Taps/jimeh"
+          ln -s "${{ github.workspace }}/tap" \
+             "$(brew --repo)/Library/Taps/jimeh/homebrew-emacs-builds"
+      - name: Checkout tap repository meta branch
+        uses: actions/checkout@v2
+        with:
+          ref: meta
+          path: meta
+      - name: Install meta dependencies
+        run: brew bundle
+        working-directory: meta
+      - name: Update meta files
+        run: make update
+        working-directory: meta
+      - uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: "chore(meta): update meta files"
+          repository: meta
+          branch: meta
+          commit_user_name: github-actions[bot]
+          commit_user_email: github-actions[bot]@users.noreply.github.com
+          commit_author: >-
+            github-actions[bot] <github-actions[bot]@users.noreply.github.com>
+          skip_dirty_check: false