Fix CI

The CI suddenly started breaking. Make it more robust by:
- Fixing Ubuntu 22.04 (this alone is not enough)
- Fixing libc++ as the C++ standard library

The latter seemed to be the actual failure. It seems like we were
calling into the include headers of libstdc++ from a system GCC
installation and that was causing build issues.

.github/workflows/build_test.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     # Run builds and tests both in Debug and RelWithDebInfo
     strategy:
         matrix:

CMakeLists.txt

@@ -12,7 +12,7 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/")
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -rdynamic")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -rdynamic -stdlib=libc++")
 set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -rdynamic -g -O0 -fsanitize=address")
 set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")