Fix Xauthority mounting on Fedora-based systems.

This patch slightly changes the way mounting XAuthority works to make it compatible with my Fedora-based (Silverblue) system. In short, XAuthority seems to be in a different location on my host system, so instead this patch mounts based on the environment variable.

(I'm not sure whether this is something you can/want to merge into master, whether we should make a branch, or whether I should just keep it in my own fork.)