CMake: Fix Overriding of CPM_SOURCE_CACHE

[HTRamsey]

Fix #12734

[HTRamsey]

@DonLakeFlyer thoughts on this? I originally didn't accommodate for the fact that CPM_SOURCE_CACHE was supposed to be an Env variable instead of a CMake variable. Also FYI passing a parameter in the terminal cmake config command will override any cache variables not marked with FORCE, so that would have still worked.

[DonLakeFlyer]

Why not just let the normal CPM stuff handle this and remove all the special stuff in CMakeLists and CustomOptions? If folks want to override they can just set the regular CPM_SOURCE_CACHE in their custom build. Is the default provided by CPM for this not good so we need to specify our own?

[HTRamsey]

Well for one I think it should be enabled by default, and also env variables are less obvious than CMake options and I want to make sure people see this option

[DonLakeFlyer]

Makes sense then