Update Dockerfile to modify Rust Makefile

Disable LLVM CI download in Rust package Makefile.

Dockerfile

@@ -6,6 +6,7 @@ FROM ghcr.io/openwrt/sdk:${SAFE_TARGET}-${VERSION}
 RUN ./scripts/feeds update packages && \
     ./scripts/feeds install rust && \
     make defconfig && \
+    sed -i 's/--set=llvm.download-ci-llvm=true/--set=llvm.download-ci-llvm=false/g' feeds/packages/lang/rust/Makefile; \
     { make package/rust/host/compile -j$(nproc) || \
       make package/rust/host/compile -j1 V=sc; } && \
     rm -rf build_dir/host/rustc-*/rustc-*/build \