Add some CI runner on top of musl libc

[ni4]

Description

Musl behaviour could differ from libstdc at some cases, so we should run tests against it as well. See the https://github.com/rnpgp/rnp/pull/1830#issuecomment-1120208929

[ronaldtse]

Agree that we should support musl which is used in replacement of glibc on Alpine.

[ronaldtse]

@maxirmx with your experience on musl, would this be an easy add? Thanks!

[maxirmx]

I think it will take 2-3 days to add the workflow. This does not count for possible compatibility issues