ExternalProject: Set environment variables

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

Modules/ExternalProject/shared_internal_commands.cmake

@@ -1896,6 +1896,7 @@ macro(_ep_get_add_keywords out_var)
     # Configure step options
     #
     CONFIGURE_COMMAND
+    CONFIGURE_ENVIRONMENT_MODIFICATION
     CMAKE_COMMAND
     CMAKE_GENERATOR
     CMAKE_GENERATOR_PLATFORM
@@ -1910,6 +1911,7 @@ macro(_ep_get_add_keywords out_var)
     # Build step options
     #
     BUILD_COMMAND
+    BUILD_ENVIRONMENT_MODIFICATION
     BUILD_IN_SOURCE
     BUILD_ALWAYS
     BUILD_BYPRODUCTS
@@ -1918,12 +1920,14 @@ macro(_ep_get_add_keywords out_var)
     # Install step options
     #
     INSTALL_COMMAND
+    INSTALL_ENVIRONMENT_MODIFICATION
     INSTALL_BYPRODUCTS
     INSTALL_JOB_SERVER_AWARE
     #
     # Test step options
     #
     TEST_COMMAND
+    TEST_ENVIRONMENT_MODIFICATION
     TEST_BEFORE_INSTALL
     TEST_AFTER_INSTALL
     TEST_EXCLUDE_FROM_MAIN