ci: antora workflow sets up clang 18

.github/workflows/ci.yml

@@ -353,7 +353,13 @@ jobs:
     name: Antora Docs
     needs: [ runner-selection ]
     runs-on: ${{ fromJSON(needs.runner-selection.outputs.labelmatrix)['ubuntu-latest'] }}
+    container: ubuntu:23.10
     steps:
+      - name: Setup Git
+        uses: alandefreitas/cpp-actions/package-install@v1.8.3
+        with:
+          apt-get: git
+
       - name: Clone Boost.URL
         uses: actions/checkout@v4
 
@@ -366,7 +372,17 @@ jobs:
         id: setup-cpp
         with:
           compiler: clang
-          version: 17
+          version: 18
+
+      - name: Set environment variables
+        run: |
+          set -x
+          clangpp_path=$(which clang++-18)
+          clang_path=$(which clang-18)
+          echo -E "CXX_COMPILER=$clangpp_path" >> $GITHUB_ENV
+          echo -E "CXX=$clang_path" >> $GITHUB_ENV
+          echo -E "C_COMPILER=$clang_path" >> $GITHUB_ENV
+          echo -E "CC=$clang_path" >> $GITHUB_ENV
 
       - name: Build Antora Docs
         run: |