default.nix: Use nixpkgs.rev if it is set

This increases the odds of automatically setting system.nixpkgsRevision,
which makes the links in the manual nice.

default.nix

@@ -10,10 +10,15 @@ let
 
   eval = evalConfig {
     inherit system;
-    modules = [ configuration ];
+    modules = [ configuration nixpkgsRevisionModule ];
     inputs = { inherit nixpkgs; };
   };
 
+  nixpkgsRevisionModule =
+    if nixpkgs?rev && lib.isString nixpkgs.rev
+    then { system.nixpkgsRevision = nixpkgs.rev; }
+    else { };
+
   # The source code of this repo needed by the [un]installers.
   nix-darwin = lib.cleanSource (
     lib.cleanSourceWith {