Merge topic 'external-project-env-mod-lists'

9cb590b843 ExternalProject: Fix lists and empty commands with environment modification
2e0a8455b3 Tests: Match expected environment modification more completely
819c6177b8 Tests: Match expected environment modification more consistently
c555e10fd4 Tests: Use shorter paths in RunCMake.ExternalProject

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !11272