Derive debian13 library dir and Rust package from the architecture

- Make the libevent install patch take CMAKE_INSTALL_LIBDIR instead of
  hardcoding lib/x86_64-linux-gnu, and rename it to
  libevent-libdir-install.patch.
- Replace every hardcoded x86_64-linux-gnu in the debian13 Dockerfile with
  $(gcc -dumpmachine).
- Download the Rust tarball for `uname -m` and expose it to the Dockerfiles
  as the architecture-neutral rust-1.99.1.tar.xz (a hard link refreshed on
  every run), because podman cannot expand ARG in a --mount source.

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

fetch-sources.sh

@@ -134,12 +134,17 @@ fetch lz4-v1.10.0.tar.gz \
     https://wangguofang.net/github.com/lz4/lz4/archive/v1.10.0.tar.gz
 fetch arrow-apache-arrow-25.0.1.tar.gz \
     https://wangguofang.net/github.com/apache/arrow/archive/apache-arrow-25.0.1.tar.gz
-fetch rust-1.99.1-x86_64-unknown-linux-gnu.tar.xz \
-    https://mirrors.cloud.tencent.com/rustup/dist/rust-1.99.1-x86_64-unknown-linux-gnu.tar.xz \
-    https://mirrors.tuna.tsinghua.edu.cn/rustup/dist/2026-10-15/rust-1.99.1-x86_64-unknown-linux-gnu.tar.xz \
-    https://repo.huaweicloud.com/rustup/dist/rust-1.99.1-x86_64-unknown-linux-gnu.tar.xz \
-    https://mirrors.ustc.edu.cn/rust-static/dist/rust-1.99.1-x86_64-unknown-linux-gnu.tar.xz \
-    https://static.rust-lang.org/dist/rust-1.99.1-x86_64-unknown-linux-gnu.tar.xz
+# Rust ships one standalone tarball per architecture, named after `uname -m` (x86_64, aarch64).
+# The Dockerfiles mount it under the architecture-neutral name rust-1.99.1.tar.xz (a hard link
+# that is refreshed on every run, so switching architectures never leaves a stale archive).
+rust_triple="$(uname -m)-unknown-linux-gnu"
+fetch "rust-1.99.1-$rust_triple.tar.xz" \
+    "https://mirrors.cloud.tencent.com/rustup/dist/rust-1.99.1-$rust_triple.tar.xz" \
+    "https://mirrors.tuna.tsinghua.edu.cn/rustup/dist/2026-10-15/rust-1.99.1-$rust_triple.tar.xz" \
+    "https://repo.huaweicloud.com/rustup/dist/rust-1.99.1-$rust_triple.tar.xz" \
+    "https://mirrors.ustc.edu.cn/rust-static/dist/rust-1.99.1-$rust_triple.tar.xz" \
+    "https://static.rust-lang.org/dist/rust-1.99.1-$rust_triple.tar.xz"
+ln -f "rust-1.99.1-$rust_triple.tar.xz" rust-1.99.1.tar.xz
 fetch duckdb-v1.5.6.tar.gz \
     https://wangguofang.net/github.com/duckdb/duckdb/archive/v1.5.6.tar.gz
 fetch duckdb-ui-0628932f74b1e2e9eaf68a05b22d737455541dca.tar.gz \