feat(cask): add workflow to trigger homebrew cask formula update on new release

This triggers the update formulas workflow in the
jimeh/homebrew-emacs-builds repo whenever a release or pre-release is
created in jimeh/emacs-builds.

This allows the cask formulas to automatically stay up to date with
latest nightly builds.

.github/workflows/dispatch-cask-update.yml

@@ -0,0 +1,15 @@
+---
+name: Dispatch Cask Update
+on:
+  release:
+    types: [published]
+
+jobs:
+  dispatch-cask-update:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Run update formulas workflow in jimeh/homebrew-emacs-builds
+        run: >-
+          gh workflow run --repo jimeh/homebrew-emacs-builds update-formulas.yml
+        env:
+          GITHUB_TOKEN: ${{ secrets.TAP_REPO_TOKEN }}