Point LLVM 21 prebuilt URLs at the upstream release

[brandonros]

Follow-up to #408. The upstream Rust-GPU/rustc_codegen_nvvm-llvm llvm-21.1.8 release is now published with all three archives, so the Linux CI fallback PREBUILT_LLVM_URL and the guide example now point at it instead of the brandonros fork. build.rs already defaulted to upstream.

🤖 Generated with Claude Code

[brandonros]

@LegNeato oops :) we probably want this

[brandonros]

Added 44ff9e0 to fix the Create manifest for Ubuntu-24.04/CUDA-13.3.1 failure on main (run):

ERROR: ghcr.io/rust-gpu/rust-cuda-ubuntu24-cuda13@sha256:5870e0e8...: not found

Cause: each build job uploads its digest as digests-<image>-<arch>, and the merge job downloads digests-<image>-*. For rust-cuda-ubuntu24-cuda13, that pattern also matches the rust-cuda-ubuntu24-cuda13-llvm21 artifacts. The download step fetched 4 artifacts instead of 2:

- digests-rust-cuda-ubuntu24-cuda13-llvm21-amd64
- digests-rust-cuda-ubuntu24-cuda13-llvm21-arm64
- digests-rust-cuda-ubuntu24-cuda13-arm64
- digests-rust-cuda-ubuntu24-cuda13-amd64

imagetools create then looked for the llvm21 digests in the rust-cuda-ubuntu24-cuda13 repo, where they don't exist. The other manifest jobs passed because no other image name starts with theirs.

Fix: put the arch first (digests-<arch>-<image>) and download digests-*-<image>. The image name is now the end of the pattern, so -llvm21 artifacts no longer match.

These steps only run on non-PR events, so this PR's CI won't test the change. The first real test is the next push to main.