git show-ref in check_remote_branch is not working at all, because the
GitHub Actions checkout fetches a git repo with only depth 1 by default.
That means only main branch is available, not any other remote branches.
As a result, git show-ref always fails.
Fix that by checking for remote branches by git ls-remote.
Changelog directory is now located under flatcar scripts, not under
coreos overlay. Fix the location to add correct changelog to the git
commit.
Also use a correct path SDK_OUTER_TOPDIR in cleanup_repo.
