Make nix ci use hyprland cachix

[fred21O4]

I noticed that the nix ci build task is not using the hyprland cachix due to it not trusting the flake substituters
This commit makes it use https://hyprland.cachix.org
This can also be done by passing --accept-flake-config instead of --substituters however I prefer to be explicit in what caches to trust.
It can be done the other way if you prefer

Describe your PR, what does it fix/add?

This makes ci builds use the hyprland cachix which will hopefully speed up builds slightly

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

This probably doesn't improve build speeds that much but it wont hurt
closed other pr 5683 I messed up the rebase

Is it ready for merging, or does it need work?

should be good to merge

[fufexan]

LGTM, thanks!