If you want to build a musl Rust project, you will need to install a musl toolchain. This can be found here:

musl.cc

You can also get a list of the available musl toolchains by running

curl -s musl.cc

Once you have selected the toolchain you want, you can get it by running

curl -O https://musl.cc/x86_64-linux-musl-cross.tgz