nix: update nixpkgs-unstable

[jcollie]

Update nixpkgs-unstable to pick up fontconfig 2.18. Currently we are linking against 2.17 and you get errors like this on standard error when using config files meant for fontconfig 2.18:

Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 20: invalid constant used :
Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 23: invalid constant used : monospace
Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 42: invalid attribute 'xsi:nil'
Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 43: invalid constant used :
Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 46: invalid constant used : sans-serif
Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 68: invalid attribute 'xsi:nil'
Fontconfig warning: "/etc/fonts/conf.d/48-guessfamily.conf", line 69: invalid constant used :

[pluiedev]

macOS looks borked? Also would be a great opportunity to replace our nixpkgs flake input to use nixpkgs.tar.zst instead of nixpkgs.tar.xz for smaller download sizes

[jcollie]

macOS looks borked? Also would be a great opportunity to replace our nixpkgs flake input to use nixpkgs.tar.zst instead of nixpkgs.tar.xz for smaller download sizes

Yeah, I've search the nixpkgs GH and they claim that the libfyaml build is supposed to be fixed on Darwin, but it's obviously not. I'll switch to the zst version the next time I push an update.