`CMAKE_POLICY_VERSION_MINIMUM` should be an environment variable

CMake 4.0.0-rc1 supports -DCMAKE_POLICY_VERSION_MINIMUM=3.5, which is insufficient because:

1. CMake projects sometimes run sub-CMake with execute_process, which does not inherit this define.
2. Some build systems are wrappers around CMake (e.g. python ecosystem) and require patching to pass further CMake command line arguments.

CMAKE_POLICY_VERSION_MINIMUM was introduced to avoid patching of existing release tarballs, so it would be more helpful if the value was initialized from an env variable.