gitnuro: Update meta.platforms

The specified jar file is only for x86-64, hence specifying that under meta.platforms.

pkgs/applications/version-management/gitnuro/default.nix

···
     description = "A FOSS Git multiplatform client based on Compose and JGit";
     homepage = "https://gitnuro.com/";
     license = licenses.gpl3Plus;
-    platforms = platforms.unix;
+    platforms = [ "x86_64-linux" ];
     maintainers = with maintainers; [ zendo ];
     mainProgram = "gitnuro";
   };