xauthority broken under ThinLinc

[Pierre Ossman (Cendio AB)]

It seems snap only handles Xauthority being in somewhat common places. It fails if it is somewhere else, forcing you to lower restrictions in your X server in some manner. There was an attempt at fixing this in this PR:

https://github.com/snapcore/snapd/pull/3177

However, it ended up only adding support for Xauthority files in /tmp.

One case where it fails is when using remote desktops via ThinLinc. The Xauthority file is then in a session specific directory under /var/opt.

E.g. on Ubuntu 22.04:

> cendio@ubuntu2204:~$ set | grep XAUTH
> XAUTHORITY=/var/opt/thinlinc/sessions/cendio/11/Xauthority
> cendio@ubuntu2204:~$ firefox
> No protocol specified
> Error: cannot open display: :11