mpvScripts.buildLua: Expose meta.position

Undocumented in the nixpkgs manual, but apparently needed for `common-updaters`
and related tools such as `nix-update`.

pkgs/applications/video/mpv/scripts/buildLua.nix

@@ -56,6 +56,17 @@ lib.makeOverridable (args: stdenvNoCC.mkDerivation (extendedBy
     '';
 
     passthru = { inherit scriptName; };
-    meta.platforms = lib.platforms.all;
+    meta = {
+      platforms = lib.platforms.all;
+    } // (
+      let pos =
+        if (args.meta or {}) ? description then
+          builtins.unsafeGetAttrPos "description" args.meta
+        else
+          builtins.unsafeGetAttrPos "pname" args;
+      in lib.optionalAttrs
+        (pos != null)
+        { position = "${pos.file}:${toString pos.line}"; }
+    );
   })
 ))