Add nixos as top-level monorepo package

I'd like to be able to call...
`nix-build -E '(import <briefcase> {}).nixos.socrates'`
...as part of my efforts to wane my dependence off of `nixos-rebuild`.

default.nix

@@ -21,6 +21,7 @@ let
   # TODO: Find a better way to expose entire monorepo without introducing
   # "infinite recursion".
   localPkgs = readTree: {
+    nixos = readTree ./nixos;
     blog = readTree ./blog;
     lisp = readTree ./lisp;
     gopkgs = readTree ./gopkgs;