Guix,

I noticed that 'tcc-boot0' from (gnu packages commencement) is
downloaded from what appears to be a GitLab-generated tarball (this is
on the 'core-updates' branch).

To ensure that any revision of Guix can be easily bootstrapped
"forever", it would be great to mirror this and adjust the URL in case
GitLab generates a new one, like we've seen GitHub do in the past.