Use $TARGETPLATFORM in Dockerfile instead of RUBY_PLATFORM

RUBY_PLATFORM corresponds to $BUILDPLATFORM, but we need $TARGETPLATFORM actually.
This is because we use the cache from the ARM64 system to build the ARM64 image on X86_64.

Author: larskanis

Dockerfile.mri.erb

@@ -25,7 +25,17 @@ COPY --from=larskanis/mingw64-ucrt:20.04 \
 RUN dpkg -i /debs/*.deb
 
 <% elsif platform =~ /aarch64-mingw-ucrt/ %>
-RUN wget https://github.com/mstorsjo/llvm-mingw/releases/download/20250114/llvm-mingw-20250114-ucrt-ubuntu-20.04-<%= RUBY_PLATFORM[/^\w+/] %>.tar.xz && \
+
+RUN if [ "$TARGETPLATFORM"="linux/amd64" ]; then \
+        DOWNLOAD_PLATFORM="x86_64"; \
+    elif [ "$TARGETPLATFORM"="linux/arm64" ]; then \
+        DOWNLOAD_PLATFORM="aarch64"; \
+    else \
+        echo "Unsupported platform $TARGETPLATFORM"; \
+        exit 1; \
+    fi && \
+    \
+    wget https://github.com/mstorsjo/llvm-mingw/releases/download/20250114/llvm-mingw-20250114-ucrt-ubuntu-20.04-$DOWNLOAD_PLATFORM.tar.xz && \
     tar xf llvm-mingw*.tar.xz && \
     mv `ls -d llvm-mingw-*/` /llvm-mingw && \
     echo "export PATH=/llvm-mingw/bin:\$PATH" >> /etc/rubybashrc && \