Merge pull request #21446 from smithfarm/wip-ctest-py3-fallback

cmake/mgr: use Python 3 virtualenv if mgr subinterpreter is Python 3

Reviewed-by: John Spray <john.spray@redhat.com>
Reviewed-by: Kefu Chai <kchai@redhat.com>

diff --cc src/pybind/mgr/dashboard/CMakeLists.txt
Simple merge