Merge topic 'test-FindPkgConfig-isolate-env' into release-3.18

8b12982653 Merge branch 'backport-test-FindPkgConfig-isolate-env'
4c939b0d39 Tests: Isolate RunCMake.FindPkgConfig from caller environment
20eeb40aaf Tests: Isolate RunCMake.FindPkgConfig from caller environment

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !5108