Make PulseAudio the default ALSA driver if available

https://github.com/ValveSoftware/steam-runtime/issues/371 indicates that PulseAudio is not the default for users of the ALSA user-space library libasound.so.2. It should be.

• In scout, check that we already install PulseAudio ALSA modules (we do)
• In soldier, install PulseAudio ALSA modules, without installing unwanted libraries
  • This probably means patching alsa-plugins in the same way Ubuntu do/did, to disable the parts that need FFmpeg (avcodec etc.), which we don't want to include
• In pressure-vessel, if the PulseAudio socket is available at runtime, create a file similar to 99-pulseaudio-default.conf.example as the container's /etc/alsa/conf.d/99-pulseaudio-default.conf (!269 (merged))
  • When not running under Flatpak, do this if flatpak_run_add_pulseaudio_args() has set PULSE_SERVER
  • When running under Flatpak, we probably want to do this unconditionally, for parity with the freedesktop.org Platform