Add the ability to modify the environment of the various steps running
as part of an external project build. This specifically adds the ability
to set them on the configure , build , install, and test steps, as well
as exposing the `ENVIRONMENT_MODIFICATION` keyword arguments to
`ExternalProject_Add_Step`, allowing customization of the environment of
custom steps.
The values of the environment variable respect the `LIST_SEPARATOR`.
Fixes: #26963
