Repository variables are not exposed to PRs from forks, so jobs that
reference vars.FLUTTER_VERSION at the container.image level fail to
resolve and the workflow template errors out before any step runs.
Replace it with a setup job that reads the latest published release tag
via the GitHub API using the read-only GITHUB_TOKEN, which is available
in fork-PR contexts.
Co-authored-by: Claude <noreply@anthropic.com>