flake.nix: Don't do cross builds in 'nix flake check'
flake.nix: Don't do cross builds in 'nix flake check'

'nix flake check' should be relatively fast, so it's not the best
place to do cross-builds. We're already doing that in Hydra.

NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix