ci: Fix Windows build and support publishing to an existing release (#6621)

* deps: Bump anstream to 0.6.21 to fix Windows build

anstream 0.6.14 declares `impl WinconStream for &'_ mut Buffer` on
Windows, which now conflicts with the blanket
`impl<T: WinconStream> WinconStream for &mut T` added in
anstyle-wincon 3.0.11. The collision breaks Windows builds with E0119.

Bumping to the semver-compatible 0.6.21 drops the explicit impl and
resolves the conflict.

* ci: Allow gnd-binary-build workflow to publish to an existing release

Adds a release_tag workflow_dispatch input. When set, the release,
publish-npm and publish-npm-wrapper jobs target the given tag instead
of github.ref_name, so a rebuilt artifact set can be attached to an
existing GitHub release and published to npm without cutting a new tag.