run: Support PulseAudio socket path without unix: prefix

https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/ServerStrings/
says:

> If the string starts with / or unix: the remaining address string is taken as
> UNIX socket name.

but previously the string was only taken to be a UNIX socket name if it began
with "unix:".

This is an issue in practice with WSLg <https://github.com/microsoft/wslg>
which sets $PULSE_SERVER to "/mnt/wslg/PulseServer", without a "unix:" prefix.

(cherry picked from commit 7534a970a5535d57d5fa07bd5ed9eb3647cbb7dc)