Support Immutable GitHub Releases

As suggested by @jtmcdole in https://github.com/flutter/skills/pull/158#discussion_r3429450916:
For another version:
- We can mark releases as immutable. GitHub will make the sha256 for us. Binaries cannot change.
- It means we upload all binaries in the release step like this:
```yaml
- name: Create Immutable GitHub Release
env:
GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
run: |
gh release create ${{ github.ref_name }} ./all-assets/* \
--title "Release ${{ github.ref_name }}" \
--generate-notes
```