ci: stop glcpp insanity
The glcpp tests have been failing and flaking for a while now, and this is a significant problem, both because it conditions everyone to stop relying on CI and because it's unbelievably annoying to have to marge things a billion times before it eventually lands.
This series reenables the build of one of the glcpp tests I disabled, adds an option to explicitly disable all glcpp tests, and disables glcpp for CI.