pinned: expose nixpkgsStable, nixpkgsUnstable

This allows accessing the pinned nixpkgs.
E.g., this is useful for comparing package versions between stable
and unstable.

pkgs/pinned.nix

@@ -21,6 +21,8 @@ in
     lightning-loop
     lightning-pool;
 
+  inherit nixpkgsStable nixpkgsUnstable;
+
   stable = nixBitcoinPkgsStable;
   unstable = nixBitcoinPkgsUnstable;
 }