pressure-vessel: Find PulseAudio socket even if no XDG_RUNTIME_DIR
On systems with no XDG_RUNTIME_DIR, PulseAudio falls back to creating a temporary directory in /tmp, then creating a symlink in legacy directory ~/.pulse or its more modern replacement ~/.config/pulse. The symlink is named according to the machine ID, falling back to the hostname.
Resolves: #51 (closed)
Resolves: https://github.com/ValveSoftware/steam-runtime/issues/343
Forwarded: https://github.com/flatpak/flatpak/pull/4059