Set RUSTFLAGS env to make dylib work

The musl-target doesn't automatically disable static linking of musl when building a dylib, and then complains it can't build a dylib.
As a workaround, disable static linking via RUSTFLAGS, to see how far the build gets.
The proper fix is to have rustc figure that out automagically.

Author: strfry

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

@@ -43,6 +43,8 @@ ENV CFLAGS_x86_64_unknown_linux_musl=-Wa,-mrelax-relocations=no
 
 ENV HOSTS=x86_64-unknown-linux-musl
 
+ENV RUSTFLAGS="-C target-feature=-crt-static"
+
 ENV SCRIPT \
       python2.7 ../x.py test --host $HOSTS --target $HOSTS && \
       python2.7 ../x.py dist --host $HOSTS --target $HOSTS