`RUSTUP_TOOLCHAIN` in `base-builder-rust:ubuntu-24-04` is older than in `base-builder-rust`

[evverx]

The ubuntu-24-04 images are currently based on the images where RUSTUP_TOOLCHAIN hasn't been bumped by analogy with 625f70d yet. Because of that attempts to switch to the new images can cause build failures/coverage mismatches.

$ podman run gcr.io/oss-fuzz-base/base-builder-rust:ubuntu-24-04 bash -xc 'rustc --version; clang --version'
+ rustc --version
rustc 1.81.0-nightly (5315cbe15 2024-07-11)
+ clang --version
clang version 22.0.0git (https://github.com/llvm/llvm-project.git cb2f0d0a5f14c183e7182aba0f0e54a518de9e3f)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin

$ podman run gcr.io/oss-fuzz-base/base-builder-rust bash -xc 'rustc --version; clang --version'
+ rustc --version
rustc 1.91.0-nightly (af00ff2ce 2025-09-04)
+ clang --version
clang version 22.0.0git (https://github.com/llvm/llvm-project.git cb2f0d0a5f14c183e7182aba0f0e54a518de9e3f)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin

[evverx]

#14245 fais because of that.