This makes `nix-build` and friends use the current flake lock instead of
the outdated pinned version we had in `./nixpkgs.nix`
With this, `nix-build -A ovni` and `nix build .#ovni` should produce the
same result.
This will fail if the flake nixpkgs input does not come from NixOS/nixpkgs.
We could use edolstra/flake-compat instead, but it's overkill imho.
Additionally, I made default.nix behave like nixpkgs, so that we can
import bscpkgs à la nixpkgs (Apply overlays and other options that nixpkgs
accepts):
```nix
let pkgs = import bscpkgs { inherit system; }; in <...>
```
Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
