[trivial] Work around failing Github actions.

There is a compatibility-issue of packages installed in the runner-image of
Ubuntu 22.04 used in our Github actions. This causes many of our automatic tests
to fail. We work around this issue by downgrading the responsible package
versions. The change should be reverted once the issue is resolved.

.github/workflows/style.yml

@@ -25,6 +25,13 @@ jobs:
           pip3 install tox
           sudo apt-get -y install clang-tidy-12
 
+      # TODO: Remove once issue with Ubuntu 22.04 and clang++-14 is resolved.
+      - name: Work around https://github.com/actions/runner-images/issues/8659
+        run: |
+          sudo rm -f /etc/apt/sources.list.d/ubuntu-toolchain-r-ubuntu-test-jammy.list
+          sudo apt-get update
+          sudo apt-get install -y --allow-downgrades libc6=2.35-* libc6-dev=2.35-* libstdc++6=12.3.0-* libgcc-s1=12.3.0-*
+
       - name: Install uncrustify
         run: |
           # Set up uncrustify.

.github/workflows/ubuntu.yml

@@ -57,6 +57,14 @@ jobs:
         run: |
           sudo apt-get -y install ${{ matrix.version.cxx }}
 
+      # TODO: Remove once issue with Ubuntu 22.04 and clang++-14 is resolved.
+      - name: Work around https://github.com/actions/runner-images/issues/8659
+        if: matrix.version.cxx == 'clang++-14'
+        run: |
+          sudo rm -f /etc/apt/sources.list.d/ubuntu-toolchain-r-ubuntu-test-jammy.list
+          sudo apt-get update
+          sudo apt-get install -y --allow-downgrades libc6=2.35-* libc6-dev=2.35-* libstdc++6=12.3.0-* libgcc-s1=12.3.0-*
+
       # Only install CPLEX if its URL/secret is set.
       - name: Install CPLEX
         if: ${{ env.CPLEX_URL != 0 }}