nix flake check: Don't build apps
nix flake check: Don't build apps

This was inconsistent since we're not building 'packages' or
'defaultPackage' either.

Closes #3726.

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