CI: Add Alpine Linux 3.23 runner to the pipeline (#18087)

Add an Alpine Linux 3.23 runner to the CI chain to run OpenZFS builds
and tests against musl libc.

Currently, zfs_send_sparse is killed after 10 minutes on Alpine, causing
cascading EBUSY failures in the test suite. With zfs_send_sparse
disabled, the ZFS test suite reaches a pass rate of 94.62%.

This commit introduces the required Alpine-specific setup and a small
set of shell and cloud-init compatibility fixes that also apply to
existing Linux runners.

The Alpine runner is not enabled by default and is not executed for new
pull requests.

Sponsored-by: ERNW Research GmbH - https://ernw-research.de/

Signed-off-by: Alexander Moch <amoch@ernw.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>

tests/zfs-tests/include/libtest.shlib

@@ -559,7 +559,7 @@ function default_cleanup_noexit
 		# Here, we loop through the pools we're allowed to
 		# destroy, only destroying them if it's safe to do
 		# so.
-		while [ ! -z ${ALL_POOLS} ]
+		while [ -n "${ALL_POOLS}" ]
 		do
 			for pool in ${ALL_POOLS}
 			do