attestation: handle bad configurations better

[Bo98]

• After Ensure early installation of gh for attestations #17760, we seem to now be forcing installing gh for everyone which I didn't initially realise based on the PR description. This can be slow for unsupported configurations as they would need to build Go from source so disable attestations for the OS.unsupported_configuration? case. (Though IMO maybe worth thinking whether we actually want to be forcing installs to 15% of users.)
• Set GH_HOST explicitly which should avoid the need for gh auth switch
• Silently handle 401 bad auth for now. Removing auth is a hard blocker for wider roll out so I don't think we should pain users in the meantime. We've been getting multiple reports a day on this issue and that number doesn't seem to be going down.

[woodruffw]

One thought, otherwise LGTM! Thanks @Bo98!

[carlocab]

I wonder if we should just skip HOMEBREW_GITHUB_API_TOKEN for attestations and rely solely on gh auth token instead. (And error only if HOMEBREW_VERIFY_ATTESTATIONS is specifically requested.)

[Bo98]

Potentially, though it's all silenced now anyway and in reality we need the auth requirement to be dropped entirely for the beta to progress any further.