Re-enable GitHub backups

[Krinkle]

Follows-up from https://github.com/jquery/infrastructure/issues/9. We have a backup role in the Puppet manifest (github::backup), and it was used for a while on the jq02 host, but that host has since been decom'ed.

This is a task to review the script, fix it if needed, and then re-enable it in one of the host roles. It's pretty minor and does not need a dedicated host. It runs once a night and needs a bit bit of space. The builder host seems like a good candidate as it's the only host with any real space needs.

[Krinkle]

I ran it just now with a personal API token of mine, and in total the cloned mirrors take up only 600M, easy!

[Krinkle]

There's an old script I removed in https://github.com/jquery/infrastructure/commit/8dfdec673dded11025f214a29e124ee787947b05 that we haven't used since 2014, but may be useful as starting point in the future.