ci: we should get rid of the magic nix cache

[cafkafk]

See: https://discourse.nixos.org/t/end-of-life-for-the-free-tier-of-the-magic-nix-cache/59306

Likely, we'll just remove this and then potentially use whatever solution I find for this at $dayjob (we'll probably not be going with private runners (which would give a persistent nix cache essentially for free) unless a gracious equinox type of thing happens, as the security implications are too annoying to worry about).