ci: add installation step for LLVM 21 in llvm-backend lane

Author: darmie

.github/workflows/ci.yml

@@ -52,6 +52,14 @@ jobs:
       - name: Cache cargo artifacts
         uses: Swatinem/rust-cache@v2
 
+      - name: Install LLVM 21 (llvm-backend lane only)
+        if: matrix.lane == 'llvm-backend'
+        run: |
+          wget -qO llvm.sh https://apt.llvm.org/llvm.sh
+          chmod +x llvm.sh
+          sudo ./llvm.sh 21 all
+          echo "LLVM_SYS_211_PREFIX=/usr/lib/llvm-21" >> "$GITHUB_ENV"
+
       - name: Build zynml for selected backend lane
         run: cargo check -p zynml ${{ matrix.cargo_features }}