ZTS: Apply small changes for speeding up the tests

The Github Action Runner got some new hardware metrics. We should use
the provided and empty disk which is pre-mounted at /mnt now.

Disk1: 89GiB -> rootfs + bootfs with ~80MB/s -> don't care
Disk2: 64GiB -> /mnt with 420MB/s -> new testing ssd

This commit will mount the new disk to /var/tmp and provide hopefully
some speedups within our testings.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Andrew Innes <andrew.c12@gmail.com>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #15811