fix(release): make the cask bump reliably runnable (#33)

The `release: published` trigger does not fire for a release whose draft was
created by tauri-action's GITHUB_TOKEN — verified on v0.8.0, which produced
zero workflow runs after publishing, so the Homebrew cask was never bumped and
the tap stayed empty.

Adds `bump_cask` to the workflow_dispatch inputs so the cask bump can be run
on demand for any tag, and switches the cask job to RELEASE_TAG (which resolves
for both dispatch and release events) instead of GITHUB_REF_NAME.

Also stops the cask template's header comment from being rewritten with the
substituted version and checksums on every render.

packaging/homebrew/gridline.rb.template

@@ -1,9 +1,9 @@
 # Homebrew Cask formula for Gridline.
 #
-# This file is the template; `scripts/render-cask.sh` substitutes
-# @VERSION@ / @ARM_SHA256@ / @INTEL_SHA256@ on each release and pushes the
-# rendered file to the `AdrianBonpin/homebrew-gridline` tap as
-# `Casks/gridline.rb`. End users then install with
+# This file is the template; `scripts/render-cask.sh` substitutes the version
+# and the per-arch SHA-256 checksums on each release and pushes the rendered
+# file to the `AdrianBonpin/homebrew-gridline` tap as `Casks/gridline.rb`.
+# End users then install with
 #
 #   brew install --cask AdrianBonpin/gridline/gridline
 #