flatpak: Unpin retro-gtk

The issue we faced has been fixed by
GNOME/retro-gtk!131.
2 jobs for unpin-retro-gtk in 42 minutes and 3 seconds
Status Name Job ID Coverage
  Test
passed flatpak #1059296
flatpak

00:42:03

passed tests #1059295
flatpak

00:06:48