chore: revert dnf. runner user has no system privs

.gitea/workflows/build-prerelease.yml

@@ -108,25 +108,12 @@ jobs:
             build_jobs: 8
             nvcc_threads: 4
             cargo_features: "cuda cudnn flash-attn"
-    # runner-cuda-13.0 inherits from runner-rust in gongfoo, so rust
-    # *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
-            dnf install -y --setopt=install_weak_deps=False rust cargo clippy
-          fi
-          cargo --version
-
       - name: Build neuron with CUDA (${{ matrix.flavour }})
         run: |
           set -eux