Update nixpkgs stable

ipimiutil broken in unstable as of 03539677da5a6caba6a6b3d2143ec63b116902d3

And on top of that `nix flake update` is crashing with

```
nix: src/libutil/posix-source-accessor.cc:13: nix::PosixSourceAccessor::PosixSourceAccessor(std::filesystem::__cxx11::path&&): Assertion `root.empty() || root.is_absolute()' failed.
Aborted (core dumped)
```

I might need to try an older or newer version...

Update / note to self:

It worked with nix from nixpkgs unstable at c77dab3ae5c71a5eaef539968ebfb13dce79b0ea

I eventually found NixOS/nix#9708 which was
for a much older issue is being used to track the new regression.
Nixpkgs downgraded back to Nix 2.24.10 in
DamienCassou/nixpkgs@0bb2579
so I'll probably just use that.