ci(prerelease): drop redundant rustup install step

The build-cortex and build-neuron jobs were running a copied-from-
mistralrs rustup install step. Both jobs use runner images that
already provide rust via dnf:

- runner-rust installs rust/cargo/clippy/rustfmt directly.
- runner-cuda-13.0 extends runner-rust.

Running 'rustup update stable' on top would install a parallel
rustup-managed toolchain and shadow the dnf one — confusing and
unnecessary. The existing ci.yml already trusts the dnf toolchain
without any install step, so match that behaviour.

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

.gitea/workflows/build-prerelease.yml

@@ -57,21 +57,14 @@ jobs:
   build-cortex:
     name: Build cortex binary
     needs: prepare
+    # runner-rust image already provides rust/cargo/clippy/rustfmt via
+    # dnf — no rustup install step needed.
     runs-on: rust
     steps:
       - uses: actions/checkout@v4
         with:
           ref: ${{ inputs.ref }}
 
-      - name: Install/update Rust toolchain
-        run: |
-          if command -v rustup &> /dev/null; then
-            rustup update stable
-          else
-            curl --proto '=https' --tlsv1.2 --silent --show-error --fail https://sh.rustup.rs | sh -s -- -y
-          fi
-          echo "${HOME}/.cargo/bin" >> "$GITHUB_PATH"
-
       - name: Build cortex (release)
         run: cargo build --release -p cortex-cli
 
@@ -115,21 +108,14 @@ 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
+    # present via dnf — no rustup install step needed.
     runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
         with:
           ref: ${{ inputs.ref }}
 
-      - name: Install/update Rust toolchain
-        run: |
-          if command -v rustup &> /dev/null; then
-            rustup update stable
-          else
-            curl --proto '=https' --tlsv1.2 --silent --show-error --fail https://sh.rustup.rs | sh -s -- -y
-          fi
-          echo "${HOME}/.cargo/bin" >> "$GITHUB_PATH"
-
       - name: Build neuron with CUDA (${{ matrix.flavour }})
         run: |
           set -eux