- build: the rootless overlay mount only exists inside podman unshare's
user namespace; run the vmlinuz check/copy and the find|cpio pipeline
there, streaming cpio's stdout to the parent for gzipping (also drops
the initrd.tmp round-trip and excludes vmlinuz from the archive)
- build: unmount the image on all code paths, not just success
- run: treat timeout's exit 124 as the expected smoke-test timeout and
error on any other non-zero status (a missing qemu binary no longer
reports success)
Fixes bug-1, bug-2, bug-3 from the code review.