testing/g4music: upgrade to 3.8.1

In 3.8, the app was renamed to Gapless, to follow the Gnome HIG.

But the repo and project still use g4music as the ID, so I think it should be OK, to keep g4music as the name:

• https://gitlab.gnome.org/neithern/g4music/-/blob/master/meson.build?ref_type=heads#L2
• https://gitlab.gnome.org/neithern/g4music/-/blob/master/pkgs/flatpak/com.github.neithern.g4music.json?ref_type=heads