flake: add devShells

exposed via nix registry :D
can be used in nix-direnv like `use flake shells#cs.osu`

modules/system/base.nix

@@ -20,6 +20,7 @@
   };
   nix.registry.config.flake = inputs.self;
   nix.registry.nixpkgs.flake = inputs.nixpkgs;
+  nix.registry.shells.flake = inputs.self;
 
   home-manager = {
     useGlobalPkgs = true;