Avoid evaluating packages twice

Previously, the packages outputs would be evalutated using callPackage
on the package set, but the package set already contains the package. If
another package depends on it, the dependency would be the version from
the package set. By grabbing the packages from the package set, each
package only needs to be evaluated once.

builtinModules/packages.nix

@@ -75,7 +75,8 @@ in
 
       outputs = rec {
         packages = genSystems (pkgs:
-          filterAttrs (_: supportedSystem pkgs) (genPkgs pkgs));
+          filterAttrs (_: supportedSystem pkgs)
+            (mapAttrs (k: _: pkgs.${k}) config.packages));
 
         checks = mapAttrs
           (_: mapAttrs' (n: nameValuePair ("packages-" + n)))