Install Rust under /usr/local instead of PREFIX

Rust is a build tool, not a library, so it should not follow the
library install prefix.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

debian13/Dockerfile

@@ -38,10 +38,11 @@ ENV PATH=${PREFIX}/bin:${PATH} \
 
 RUN test "$(cmake --version | sed -n '1s/[^0-9]*//p' | cut -d. -f2)" -ge 29
 
+# Rust is a build tool, not a library, so it is installed under /usr/local regardless of ${PREFIX}.
 RUN --mount=type=bind,from=sources,source=rust,target=/tmp/rust \
     mkdir rust-src \
     && tar -xJf "/tmp/rust/rust-1.99.1-$(uname -m)-unknown-linux-gnu.tar.xz" -C rust-src --strip-components=1 \
-    && rust-src/install.sh --prefix=${PREFIX} --without=rust-docs \
+    && rust-src/install.sh --prefix=/usr/local --without=rust-docs \
     && rm -rf rust-src
 
 RUN --mount=type=bind,from=sources,source=boost_1_83_0.tar.bz2,target=/tmp/boost.tar.bz2 \