[release-24.05] Update pinned nixpkgs version

[infinisil]

This is necessary for #327796, the backport of #322537, to work, because:

• The 24.05 branch enforces (some) Nix files to be formatted with the nixfmt-rfc-style version from the CI-pinned Nixpkgs version.
• In order for automated backports to 24.05 to succeed after the reformat, we need to use the same formatter version on all the branches, otherwise you get conflicts due to different formattings.

So this backports all the commits that updated the CI version on master and 24.11: #336555 #361165 #363585

Ping @NixOS/nix-formatting

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This work is funded by Tweag and Antithesis ✨

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[infinisil]

Reviewed in a call with lots of others together, no complaints came up, so let's merge :)