CI: Add logs to zloop workflow

On failure attempt to include the most relevant portions of the
ztest logs in the CI output. This full logs are still available
for download but often a backtrace and the last output is enough.

Install libunwind to improve the odds of a useful backtrace.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16573