gnome/nixos: use mkTarget

[0xda157]

Things done

• [ ] Tested locally
• [ ] Tested in testbed
• [x] Commit message follows commit convention
• [ ] Fits style guide
• [ ] Respects license of any existing code used

Notify maintainers

@danth

[0xda157]

@MattSturgeon infinite recursion here too, could you take a look?

[0xda157]

I don't see any obvious inf-rec here, and CI seems happy atm?

see https://github.com/nix-community/stylix/actions/runs/15172177179/job/42664658225#step:5:18487