Build cmake earlier on dist-x86_64-musl

musl-toolchain.sh is called with REPLACE_CC=1, so it will replace
the host compiler and the subsequent cmake build will fail because
it cannot find the openssl headers.

Move the cmake build earlier, so it happens before the compiler
is replaced.

Author: nikic

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

@@ -20,6 +20,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 
 WORKDIR /build/
 
+# Build cmake before musl toolchain, as we replace the compiler during that step.
+COPY scripts/cmake.sh /scripts/
+RUN /scripts/cmake.sh
+
 COPY scripts/musl-toolchain.sh /build/
 # We need to mitigate rust-lang/rust#34978 when compiling musl itself as well
 RUN CFLAGS="-Wa,-mrelax-relocations=no -Wa,--compress-debug-sections=none -Wl,--compress-debug-sections=none" \
@@ -29,9 +33,6 @@ RUN CFLAGS="-Wa,-mrelax-relocations=no -Wa,--compress-debug-sections=none -Wl,--
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
-COPY scripts/cmake.sh /scripts/
-RUN /scripts/cmake.sh
-
 ENV HOSTS=x86_64-unknown-linux-musl
 
 ENV RUST_CONFIGURE_ARGS \