Mount the Rust archive from its own sources/rust directory

Download the per-architecture Rust tarball into sources/rust/ and have the
rocky9 and debian13 Dockerfiles bind that directory and pick the archive
with uname -m. This drops the architecture-neutral hard link and keeps the
Rust layer's cache key independent of other source archives.

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

fetch-sources.sh

@@ -135,16 +135,16 @@ fetch lz4-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
 # 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).
+# 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="$(uname -m)-unknown-linux-gnu"
-fetch "rust-1.99.1-$rust_triple.tar.xz" \
+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" \
     "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 \