base-builder-rust: bump RUSTUP_TOOLCHAIN to nightly-2025-09-05 on ubuntu-24-04

The default base-builder-rust Dockerfile was bumped to nightly-2025-09-05
in 625f70d, but the ubuntu-24-04 variant
was left at the older nightly-2024-07-12, causing build failures and
coverage mismatches when projects opt into ubuntu-24-04 (#14220).

The LLVM revision (cb2f0d0a5f14) is now identical across the default,
ubuntu-20-04 and ubuntu-24-04 base-clang scripts, so the toolchain/LLVM
compatibility concern that originally delayed this bump no longer
applies.

Fixes: #14220

Author: masumi-ryugo

infra/base-images/base-builder-rust/ubuntu-24-04.Dockerfile

@@ -26,7 +26,7 @@ ENV OSSFUZZ_RUSTPATH /rust
 # manually specifying what toolchain to use. Note that this environment variable
 # is additionally used by `install_rust.sh` as the toolchain to install.
 # cf https://rust-lang.github.io/rustup/overrides.html
-ENV RUSTUP_TOOLCHAIN nightly-2024-07-12
+ENV RUSTUP_TOOLCHAIN nightly-2025-09-05
 
 # Configure the linker used by default for x86_64 linux to be `clang` instead of
 # rustc's default of `cc` which is able to find custom-built libraries like