mirror-nixos-branch: also expose date of commit, show timezone to avoid confusion

[emilylange]

As requested in NixOS/infra#797, follow up to #76.

I manually tested the relevant bits of mirror-nixos-branch.pl by patching various variables, conditionals and some help of print and die.

The change to the redirect URL of nixexprs.tar.xz was tested with the help of a local reverse proxy.

^{PS: I do not like Perl}

[mweinelt]

Thanks, that looks great!

[vcunat]

🤔 I don't really mind, but why print the commit hash again? I see it five times in there now:

Starting Update Channel nixos-unstable-small...
 $ git fetch origin 33dee0014543abf6ada764a776fa0f93fbbe325c >&2
From https://github.com/NixOS/nixpkgs
 * branch                      33dee0014543abf6ada764a776fa0f93fbbe325c -> FETCH_HEAD
Release information:
 - release is: nixos-25.11pre898967.33dee0014543 (build 314081203)
 - eval is: 1820330
 - prefix is: nixos/unstable-small/nixos-25.11pre898967.33dee0014543
 - Git commit is: 33dee0014543abf6ada764a776fa0f93fbbe325c
 - Git commit date is: 2025-11-20 12:10:37 UTC
previous release is https://releases.nixos.org/nixos/unstable-small/nixos-25.11pre898540.094318ea1650
downloading [...]

[emilylange]

I felt like the git fetch output is worthwhile in situation when e.g. GitHub is having issues again.
But sure, we could add --quiet as argument to git fetch or discard the output by some other way.

This feedback is appreciated, thanks. And I am open to change this.

It's mostly that I have no idea how those logs are used besides some rare debugging sessions.
So I went with slightly more output than absolutely necessary in an attempt to every so slightly aid those debugging sessions.

PS: Also, just to avoid confusion for future readers seeing the diff of this PR here: The git fetch output is caused by the follow-up PR #101 and not this one.