The std module can be tested in the generic C++23 build. This removes the
special module build.
Note it is possible to enable modules automatically in CMake, but that
would fail in the "parts disabled" builds; they have not been properly
been converted to modules yet.
Depends on D157625
Instead of adding a new job with std modules disabled, I would instead do this:
Then it becomes more clear that this is a temporary option until it can be enabled by default everywhere (also, please document that env var as such so that we eventually clean it up).