Don't install zstd until after the first LLVM build.

Author: khuey

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

@@ -56,16 +56,16 @@ RUN ./build-gcc.sh && yum remove -y gcc gcc-c++
 COPY scripts/cmake.sh /tmp/
 RUN ./cmake.sh
 
-# rustc's LLVM needs zstd.
-COPY scripts/zstd.sh /tmp/
-RUN ./zstd.sh
-
 # Now build LLVM+Clang, afterwards configuring further compilations to use the
 # clang/clang++ compilers.
 COPY host-x86_64/dist-x86_64-linux/build-clang.sh /tmp/
 RUN ./build-clang.sh
 ENV CC=clang CXX=clang++
 
+# rustc's LLVM needs zstd.
+COPY scripts/zstd.sh /tmp/
+RUN ./zstd.sh
+
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh