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-5-setup.sh

@@ -58,13 +58,21 @@ for ((i=1; i<=VMs; i++)); do
 fqdn: vm$i
 
 users:
-- name: root
-  shell: $BASH
-- name: zfs
-  sudo: ALL=(ALL) NOPASSWD:ALL
-  shell: $BASH
-  ssh_authorized_keys:
-    - $PUBKEY
+  - name: root
+    shell: /bin/bash
+    sudo: ['ALL=(ALL) NOPASSWD:ALL']
+  - name: zfs
+    shell: /bin/bash
+    sudo: ['ALL=(ALL) NOPASSWD:ALL']
+    ssh_authorized_keys:
+      - $PUBKEY
+    # Workaround for Alpine Linux.
+    lock_passwd: false
+    passwd: '*'
+
+packages:
+  - sudo
+  - bash
 
 growpart:
   mode: auto