FindLua: Set CMP0012 to NEW for the revised search code

Code added by commit v3.12.0-rc1~53^2 (FindLua: Search for lua.h using
more conventional paths, 2018-05-20) depends on `CMP0012` NEW behavior.
Set the policy explicitly for the scope of the FindLua module.

Fixes: #18142
17 jobs for fix-lua
Status Name Job ID Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager