lib/types: only accept derivations for shellPackage

Since shellPackage actually requires the value to be an attribute set
(i. e. an derivation in this case), we cannot re-use the package.check
type checker since it also allows strings or things that are coercible
to strings as long as they look like store paths.

lib/types.nix

@@ -337,7 +337,7 @@ rec {
     };
 
     shellPackage = package // {
-      check = x: (package.check x) && (hasAttr "shellPath" x);
+      check = x: isDerivation x && hasAttr "shellPath" x;
     };
 
     path = mkOptionType {