Please add a MUSL build to avoid GLIBC version problems

Please add `postgrestools_aarch64-unknown-linux-musl` and `postgrestools_x86_64-unknown-linux-musl` builds in the release.

This helps the binary distribution to work not just on MUSL Linux systems but also on older GNU/Linux like Ubuntu 20.04 (which poor me is using, and getting ``version `GLIBC_2.34' not found``).