fix: Use nixpkgs-fmt from nixpkgs
nixpkgs-fmt v1.2.0 doesn't seem to be installable in CI right now: https://gitlab.com/victor-engmark/tilde/-/jobs/1781421718.
nixpkgs-fmt v1.2.0 doesn't seem to be installable in CI right now: https://gitlab.com/victor-engmark/tilde/-/jobs/1781421718.