ci: add build with just gtest, eigen that uses C++20

[fnrizzi]

• tpls needed: gtest, Eigen 3.4

• compiler supporting C++20, ideally both clang and gcc

• need to make docker images for this setup, start from these ones

• need to upload the new images to https://hub.docker.com/search?q=pressiomodelreduction (I will share password)

• cmake line to build pressio:

  cmake -D CMAKE_BUILD_TYPE:STRING=Release -D CMAKE_VERBOSE_MAKEFILE:BOOL=TRUE -D CMAKE_C_COMPILER:FILEPATH=/usr/local/Cellar/gcc@11/11.3.0/bin/gcc-11 -D CMAKE_CXX_COMPILER:FILEPATH=/usr/local/Cellar/gcc@11/11.3.0/bin/g++-11 -D PRESSIO_ENABLE_TPL_EIGEN=ON -D EIGEN_INCLUDE_DIR=<path-to-eigen> -D GTEST_ROOT=<path-to-gtest-install> -D PRESSIO_ENABLE_DEBUG_PRINT=ON -D PRESSIO_ENABLE_TESTS:BOOL=ON -D CMAKE_INSTALL_PREFIX:PATH=../install  -D CMAKE_CXX_STANDARD=20 -S <source>