Fetch commit date instead of last eval time

[PerchunPak]

This fixes the bug where status.nixos.org would show the date of the last successes eval instead of the more informative commit date.

Current status.nixos.org:

After this change:

[collares]

The current timestamp in status.nixos.org is when the channel last advanced, right? To me, that fits the "Last updated" header better than the timestamp of the corresponding commit (which is easily obtained by clicking the commit SHA). Is there another place which currently displays when a channel last advanced?

[adamcstephens]

Why not add a second column for commit age?

[vcunat]

To me personally stamp of the channel commit is much more useful than "last updated" stamp. (Ideally I'd want to see the push stamp of the next commit after the channel, but that would be too complex I guess.)

[vcunat]

This had been discussed e.g. on #7

[PerchunPak]

I'm unsure what was 5 years ago, but currently all channel heads are merge commits that have a timestamp, when a PR was merged. AFAIK, no one has direct push access to nixpkgs and so the update time must be accurate.

Please see this discussion for more context
https://matrix.to/#/!kjdutkOsheZdjqYmqp:nixos.org/$aG-TWSnYNDd-SDqP7mgPg0ReBT8svudHFkJUOlAkVWY?via=nixos.org&via=matrix.org&via=tchncs.de

[vcunat]

Right. Some direct pushes were there at that point, but commit time is normally refreshed on any cherry-pick or rebase, so I don't think this aspect makes a big difference. (I think most were merges, too.)