Map macOS arm64 to aarch64 when fetching the Rust archive

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

fetch-sources.sh

@@ -139,7 +139,10 @@ fetch arrow-apache-arrow-25.0.1.tar.gz \
 # Rust ships one standalone tarball per architecture, named after `uname -m` (x86_64, aarch64).
 # It lives in its own sources/rust/ directory, which the Dockerfiles mount as a whole and pick the
 # archive from by architecture, so adding other sources never invalidates the Rust build layer.
-rust_triple="$(uname -m)-unknown-linux-gnu"
+rust_arch="$(uname -m)"
+# macOS reports arm64 where Linux (and Rust) call it aarch64.
+[ "$rust_arch" = arm64 ] && rust_arch=aarch64
+rust_triple="$rust_arch-unknown-linux-gnu"
 mkdir -p rust
 fetch "rust/rust-1.99.1-$rust_triple.tar.xz" \
     "https://mirrors.cloud.tencent.com/rustup/dist/rust-1.99.1-$rust_triple.tar.xz" \