Merge topic 'fetchcontent-reject-CMAKE_TOOLCHAIN_FILE-env-var'

c15674a9cb FetchContent: Reject CMAKE_TOOLCHAIN_FILE env var in sub-build
31ef93f19f FetchContent: Only use @-@ replacements for sub-build CMakeLists.txt

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !8248