update-scripts: add "version-info" and run it in update CI

[MattSturgeon]

• update-scripts: get nixpkgs using flak-compat
• update-scripts/version-info: init
• update-scripts: move update logic from CI to dedicated script
• update-scripts/update: write version-info during update
• version-info: init

[GaetanLepage]

Looks good overall. Is this to later use nixvimOlder & co?

[MattSturgeon]

Is this to later use nixvimOlder & co?

I've ran into wanting version info a few times. Sometimes it'd be acceptable to use lib.trivial.release, but other times we need to know the "definitely not modified by a follows" info.

I didn't want this PR to grow into also implementing various use-cases for version info.

Some uses include minor improvements to the docs, throwing assertions when the wrong nixpkgs channel is used, improving nixvim's "branching off a release" process, etc.

There's probably other version info worth tracking in the version-info.toml file too; or alternatively introducing a lib.nixvim.version that has a mixture of static and dynamic values? Either way, that's future work.

[mergify]

This pull request, with head sha 46fd0b184cbc5f1bdc5a8325cb973fc54e49ab68, has been successfully merged with fast-forward by Mergify.

This pull request will be automatically closed by GitHub.

As soon as GitHub detects that the sha 46fd0b184cbc5f1bdc5a8325cb973fc54e49ab68 is part of the main branch, it will mark this pull request as merged.

It is possible for this pull request to remain open if this detection does not happen, this usually happens when a force-push is done on this branch version-info, this means GitHub will fail to detect the merge.

[nixvim-ci]

Successfully created backport PR for nixos-25.05:

• [Backport nixos-25.05] update-scripts: add "version-info" and run it in update CI #3423