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>

.github/workflows/scripts/qemu-6-tests.sh

@@ -95,11 +95,18 @@ case "$1" in
     ;;
 esac
 
-# enable io_uring on el9/el10
+# Distribution-specific settings.
 case "$1" in
   almalinux9|almalinux10|centos-stream*)
+    # Enable io_uring on Enterprise Linux 9 and 10.
     sudo sysctl kernel.io_uring_disabled=0 > /dev/null
     ;;
+  alpine*)
+    # Ensure `/etc/zfs/zpool.cache` exists.
+    sudo mkdir -p /etc/zfs
+    sudo touch /etc/zfs/zpool.cache
+    sudo chmod 644 /etc/zfs/zpool.cache
+    ;;
 esac
 
 # run functional testings and save exitcode