resolve `unknown flake output 'schemas' warning

[maxheld83]

nix flake check gives:

warning: unknown flake output 'schemas'

GHA and locally

[maxheld83]

this problem is still present

[maxheld83]

this is caused by schemas not having landed upstream in nix.
So this is blocked until NixOS/nix#8892 gets merged, or it gets front-run into determinate nix.