ci: make "linux-musl" job use zlib-ng

ci: make "linux-musl" job use zlib-ng

We don't yet have any test coverage for the new zlib-ng backend as part
of our CI. Add it by installing zlib-ng in Alpine Linux, which causes
Meson to pick it up automatically.

Note that we are somewhat limited with regards to where we run that job:
Debian-based distributions don't have zlib-ng in their repositories,
Fedora has it but doesn't run tests, and Alma Linux doesn't have the
package either. Alpine Linux does have it available and is running our
test suite, which is why it was picked.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>