Add `x86_64-unknown-linux-musl` to `dist` configuration and to GitHub Actions CI matrix

[alilleybrinker]

We currently only do CI testing and release binary builds for x86 Linux with glibc, running on Ubuntu 22.04. This means our pre-built binaries expect to be able to dynamically link with glibc when run, and aren't standalone, and also that they won't run on Linux hosts which do not have glibc. Alpine is the most popular non-glibc-bundling Linux distro I'm aware of, but of course there are others.

dist makes it pretty easy to add support for musl libc, with the x86_64-unknown-linux-musl target triple in the platforms array.

It may be slightly more complicated to add the same support in our regular CI builds. Of course, we'd need to do both at the same time to ensure we don't let build breakage slip in during development and then only catch it during release automation. Release automation failures generally require tagging a new patch version to trigger fresh automation runs, which isn't ideal.