The opaque `CMAKE_SYSTEM_ENVIRONMENT_ID` variable is cached during the
first invocation of `cmake` and is a user-defined hint about the state
of the environment. When the value changes, `cmake` either does nothing,
emits a warning (by default), or refreshes the cache as if by `--fresh`.
The action is controlled by the `CMAKE_SYSTEM_ENVIRONMENT_ACTION`
environment variable (which is not cached).
Implements: #28025
