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/zfs-qemu.yml

@@ -10,6 +10,11 @@ on:
         required: false
         default: ""
         description: "(optional) Experimental kernel version to install on Fedora (like '6.14' or '6.13.3-0.rc3')"
+      specific_os:
+        type: string
+        required: false
+        default: ""
+        description: "(optional) Only run on this specific OS (like 'fedora42' or 'alpine3-23')"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
@@ -58,6 +63,9 @@ jobs:
               # They specified a custom kernel version for Fedora.
               # Use only Fedora runners.
               os_json=$(echo ${os_selection} | jq -c '[.[] | select(startswith("fedora"))]')
+          elif ${{ github.event.inputs.specific_os != '' }}; then
+              # Use only the specified runner.
+              os_json=$(jq -cn --arg os "${{ github.event.inputs.specific_os }}" '[ $os ]')
           else
               # Normal case
               os_json=$(echo ${os_selection} | jq -c)