Building with GTest doesn't work due to recent release

GTest released version 12 on June 22nd, which caused the pipeline build to fail (as the pipeline always clones the latest commit on main branch). Te latest gtest commit working that was used in TopGen is 191ca1f3a9262b90a586ae2c2e8c742c3d867801 (dated June 14th). When hardcoding a checkout to this commit in the GitLab pipeline runner, build job succeeds.