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

[masumi-ryugo]

Fixes #14220.

The default base-builder-rust Dockerfile was bumped to nightly-2025-09-05 in 625f70d, but the ubuntu-24-04 variant was left at nightly-2024-07-12. As reported in #14220, this causes build failures / coverage mismatches when projects opt into the ubuntu-24-04 image (currently 360 projects).

This PR aligns ubuntu-24-04.Dockerfile with the default Dockerfile (1-line change).

Why it's safe to bump now

The original concern raised on #13964 was that RUSTUP_TOOLCHAIN couldn't be bumped on the new images while LLVM was at a different revision. As of master, OUR_LLVM_REVISION=cb2f0d0a5f14 is identical across all three base-clang scripts:

infra/base-images/base-clang/checkout_build_install_llvm.sh:OUR_LLVM_REVISION=cb2f0d0a5f14
infra/base-images/base-clang/checkout_build_install_llvm_ubuntu_20_04.sh:OUR_LLVM_REVISION=cb2f0d0a5f14
infra/base-images/base-clang/checkout_build_install_llvm_ubuntu_24_04.sh:OUR_LLVM_REVISION=cb2f0d0a5f14

So the toolchain/LLVM compatibility concern that delayed this no longer applies — nightly-2025-09-05 already runs successfully against this LLVM revision in the default image.

Scope

• Only ubuntu-24-04.Dockerfile is touched (the file the issue is about).
• ubuntu-20-04.Dockerfile is also at nightly-2024-07-12 but Ubuntu 20.04 reached EOL in April 2025; whether to bump or remove that file is a separate decision and is intentionally out of scope here.

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[masumi-ryugo]

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