flatpak: Use the BlastEm core from Libretro

It should be in a good enough shape to be used now.

Fixes https://gitlab.gnome.org/GNOME/gnome-games/-/issues/74
2 jobs for blastem in 27 minutes and 55 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed flatpak #1041935
flatpak

00:27:55

passed tests #1041934
flatpak

00:11:29