get-archive: check proper HTTP Content-Type
Out GitLab silently redirects certain invalid URLs to New Issue page while returning 200.
Check the HTTP response Content-Type when saving archive in order not to save HTML pages instead of archive data.
Testing URL:
https://gitlab.nic.cz/knot/COVFEFE/-/archive/v5.2.1/knot-resolver-v5.2.1.tar.gz