[MLIR] [CMake/Python] Fix ignored Python3_EXECUTABLE

On Windows (at least), cmake ignores Python3_EXECUTABLE unless the
'Interpreter' component is being found. If the user is specifying a
different version than the latest installed (say, 3.8 vs 3.9) with the
Python3_EXECUTABLE, cmake was using a combination of the newest version
and the desired version. Mitigated by adding 'Interpreter' in the first
invocation like the second one.