Since:

* commit e216b9bb (cmake: Allow CMAKE_BUILD_TYPE to be set by
                     environment variable, 2021-06-29, v3.22.0-rc1~503^2~1)
* commit ef56eefc (cmake: Allow CMAKE_CONFIGURATION_TYPES to be set by
                     environment variable, 2021-06-29, v3.22.0-rc1~503^2)

the environment variables are supposed to provide defaults for settings
the user otherwise can control via cache entries.  However, they
accidentally affect `try_compile` projects too, which are supposed to be
programmatically controlled.

Fixes: #22935