enable llvm.libzstd on test x64 linux builder

Author: lqd

src/ci/docker/host-x86_64/x86_64-gnu/Dockerfile

@@ -28,5 +28,6 @@ ENV RUST_CONFIGURE_ARGS \
  --build=x86_64-unknown-linux-gnu \
  --enable-sanitizers \
  --enable-profiler \
- --enable-compiler-docs
+ --enable-compiler-docs \
+ --set llvm.libzstd=true
 ENV SCRIPT python3 ../x.py --stage 2 test