Add an "unstable" aggregate to replace the "unstable" view

Views are obsolete, aggregates are the declarative replacement.

pkgs/top-level/make-tarball.nix

@@ -2,9 +2,7 @@
    also builds the documentation and tests whether the Nix expressions
    evaluate correctly. */
 
-{ nixpkgs ? { outPath = (import ./all-packages.nix {}).lib.cleanSource ../..; revCount = 1234; shortRev = "abcdef"; }
-, officialRelease ? false
-}:
+{ nixpkgs, officialRelease }:
 
 with import nixpkgs.outPath {};