Enable docs on dist-x86_64-musl

Add the rust-docs component to toolchain x86_64-unknown-linux-musl, which allows
people using rustup on their musl-based linux distribution to download the
rust-docs.

Author: David Sonder

src/ci/docker/host-x86_64/dist-x86_64-musl/Dockerfile

@@ -33,7 +33,6 @@ ENV HOSTS=x86_64-unknown-linux-musl
 ENV RUST_CONFIGURE_ARGS \
       --musl-root-x86_64=/usr/local/x86_64-linux-musl \
       --enable-extended \
-      --disable-docs \
       --enable-lld \
       --set target.x86_64-unknown-linux-musl.crt-static=false \
       --build $HOSTS