_7zz: fix passing package to tests

pkgs/tools/archivers/7zz/default.nix

···
 , enableUnfree ? false
 
   # For tests
-, _7zz
 , testers
 }:
 
···
   passthru = {
     updateScript = ./update.sh;
     tests.version = testers.testVersion {
-      package = _7zz;
+      package = finalAttrs.finalPackage;
       command = "7zz --help";
     };
   };