Evaluate exported pkgs against repo baseline nixos

The baseline pkgs for own consumption is nixos, only pkgs/override.nix
provides a mechanism to "backport" packages from unstable.

Therefore, we should expose our packages to others in the same
context as we use them ourselves.

flake.nix

@@ -64,7 +64,7 @@
         let
           pkgs = pkgImport {
             inherit system;
-            pkgs = master;
+            pkgs = nixos;
             overlays = [ devshell.overlay ];
           };