'musl' environment should not hardcode static linking

For a while now, in rust, whether linking is static or dynamic is controlled by the -Ccrt-static, rather than being hardcoded in the target (although the target still sets the default value). Perhaps the CARGO_CFG_TARGET_FEATURE variable should be read, as Alpine, Gentoo and Void all patch rust to enable dynamic linking by default, and upstream Rust is planning to do the same.