fix(ci): ensure rust toolchain present on cuda-13.0 runner

The currently-published runner-cuda-13.0 image (gongfoo) is missing
rust/cargo despite inheriting from runner-rust. Build-neuron fails
immediately with 'cargo: command not found' even though build-cortex
on the bare 'rust' runner builds fine.

Add a defensive `dnf install rust cargo clippy` step at the top of
build-neuron. Idempotent — on a properly-built runner image this is
a fast no-op; on the current broken image it installs the toolchain
in a few seconds. The runner image itself should be rebuilt in
gongfoo so this step becomes redundant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-prerelease.yml

@@ -108,14 +108,25 @@ jobs:
             build_jobs: 8
             nvcc_threads: 4
             cargo_features: "cuda cudnn flash-attn"
-    # runner-cuda-13.0 extends runner-rust, so rust/cargo are already
+    # runner-cuda-13.0 inherits from runner-rust in gongfoo, so rust
-    # present via dnf — no rustup install step needed.
+    # *should* be available via dnf. The currently-published image is
+    # missing it though (likely a stale build), so we run a defensive
+    # `dnf install` at the top of the step. When the runner image is
+    # rebuilt with the proper layers this becomes a fast no-op.
     runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
         with:
           ref: ${{ inputs.ref }}
 
+      - name: Ensure rust toolchain present
+        run: |
+          set -eux
+          if ! command -v cargo >/dev/null 2>&1; then
+            sudo dnf install -y --setopt=install_weak_deps=False rust cargo clippy
+          fi
+          cargo --version
+
       - name: Build neuron with CUDA (${{ matrix.flavour }})
         run: |
           set -eux