zshenv: switch default mount point from /lib/live/mount/medium to /run/live/medium

zshenv: switch default mount point from /lib/live/mount/medium to /run/live/medium

In commit 0d878d3a679 of live-boot(-grml)
("Simplify mount point handling by using /run/live instead of /lib/live/mount")
the mountpath of /lib/live/mount/medium was moved towards /run/live/medium.

Commit c6a17c7b41b of live-boot(-grml) provides a backward compatibility
rbind mount, but occasionally there seems to be a regression somewhere
during boot (see https://github.com/grml/live-boot-grml/issues/10), and
the rbind mount will be deprecated and removed before the bullseye
(Debian 11) release.

Layout changes over time:

* /cdrom for old linuxrc approach
* /live/image for initramfs layout until December 2012
* /lib/live/mount/medium for initramfs layout since December 2012
* /run/live/medium for initramfs layout since December 2018