Fix issue 11892

It seems that I copied the expression for baseDir thoughtlessly and
did not come back to it.

- `baseDir` was only used in the `fromArgs` branch.
- `fromArgs` is true when `packages` is true.

tests/functional/shell.nix

@@ -37,7 +37,7 @@ let pkgs = rec {
       mkdir -p $out
       ln -s ${setupSh} $out/setup
     '';
-  };
+  } // { inherit mkDerivation; };
 
   shellDrv = mkDerivation {
     name = "shellDrv";
@@ -94,5 +94,9 @@ let pkgs = rec {
     chmod a+rx $out/bin/ruby
   '';
 
+  inherit (cfg) shell;
+
+  callPackage = f: args: f (pkgs // args);
+
   inherit pkgs;
 }; in pkgs