CI: Add Alpine (musl)

CI: Add Alpine (musl)

I'm not intending to add every single possible combination of distros
to compiler-zoo, but I think this one is worthwhile.

musl tends to be Different Enough (TM) to allow problems to be found,
in particular (but not limited to) its malloc implementation ("mallocng").

It's also quite a common environment, especially in containers, so
I think it's worth testing on.

Signed-off-by: Sam James <sam@gentoo.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19577)