Fix failing Clang builds on GitHub Actions

A while ago the 'ubuntu-latest' image moved from Ubuntu 22.04 to
Ubuntu 24.04. See <actions/runner-images#10636>.
However, Ubuntu 24.04 does not have packages for Clang 11 and 12
anymore, thus the Clang jobs failed when they tried to install
the corresponding compilers. Therefore, the workflow is changed
to use 'ubuntu-22.04' instead of 'ubuntu-latest'.

.github/workflows/ci.yml

@@ -47,9 +47,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: [11, 12]
+        include:
+          - version: 11
+            os: 'ubuntu-22.04'
+          - version: 12
+            os: 'ubuntu-22.04'
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v4
@@ -61,7 +65,7 @@ jobs:
         env:
           CXX: clang-${{ matrix.version }}
         run: cmake -S . -B build
-          -D CMAKE_CXX_COMPILER=clang++
+          -D CMAKE_CXX_COMPILER=clang++-${{ matrix.version }}
           -D CMAKE_BUILD_TYPE:STRING=Release
           -D ${{ env.PROJECT }}_OPT_SELECT_NONSTD=ON
           -D ${{ env.PROJECT }}_OPT_BUILD_TESTS=ON