Work around issue with github linux runners

Something happened where compilation starting
failing in Linux. This is due to some change
in the VMs github uses to run these builds.
Will probably have to remove this at some point?

More details in workflow.

.github/workflows/cmake_ctest.yml

@@ -52,6 +52,9 @@ jobs:
       # Thanks to McMartin & co https://forum.juce.com/t/list-of-juce-dependencies-under-linux/15121/44
       run: |
         sudo apt-get update && sudo apt install libasound2-dev libx11-dev libxinerama-dev libxext-dev libfreetype6-dev libwebkit2gtk-4.0-dev libglu1-mesa-dev xvfb fluxbox ninja-build
+        # downgrade gcc to workaround 22.04 and C++20 issue
+        # see: https://github.com/actions/runner-images/issues/8659
+        sudo apt-get install -y --allow-downgrades libc6=2.35-0ubuntu3.4 libc6-dev=2.35-0ubuntu3.4 libstdc++6=12.3.0-1ubuntu1~22.04 libgcc-s1=12.3.0-1ubuntu1~22.04
         sudo /usr/bin/Xvfb $DISPLAY &
 
     # This lets us use sscache on Windows