Change default CMAKE_BUILD_TYPE to Release

Currently QtSetup.cmake checks if there's a ".git" dir and defaults to CMAKE_BUILD_TYPE=Debug in such a case.

That's kinda weird considering it somewhat overlaps with FEATURE_developer_build.

We should probably remove the .git check, and just default to Release.

Explicitly opting into FEATURE_developer_build should set it to Debug.

This may or may not have some consequences in the CI, per-repo builds, and standalone tests builds.

So care should be taken to check all those cases.