Add CMAKE_POLICY_VERSION_MINIMUM environment variable

Extend commit 1a35351125 (Add CMAKE_POLICY_VERSION_MINIMUM to help
configure outdated projects, 2025-02-13, v4.0.0-rc1~12^2) with an
environment variable to initialize the cache entry.  That will make it
easier to use when `cmake` is invoked under layers of scripting.

Closes: #26715