nixos: export packages of the current configuration

[oxij]

Motivation for this change

A trivial patch I frequently miss when testing SLNOS patches with upstream nixpkgs.

This allows one to access a package configured with overrides given in nixpkgs.config, e.g.:
nix-build ./nixos/default.nix -A pkgs.ffmpeg

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[oxij]

Thanks!