Better handling of fallback not existing (duplicate, not needed)

[DerekRoberts]

If tag_fallback isn't specified it will trigger the build. So there is no need to check if that tag exists.

Changed the way the fallback image check works. Now The command is run with all output and error sent to /dev/null. Then using the $? variable which will be 0 if found or 1 for other errors.

GitHub has a quirk. If NO packages exist for a repository it returns 403 instead of 401 not found.

Thanks @pauldaustin for the PR! GitHub Actions permissions don't allow package writing from forks, so the branch has been copied here.

[DerekRoberts]

Never mind! Original PR was fixable.