Check early for PythonInterp 2 in CMakeLists.txt

Check early for PythonInterp 2 in CMakeLists.txt

SPIRV-Tools looks for a Python without specifying the version, which
causes PYTHON_EXECUTABLE to reference a Python 3 executable even if
later code needs PythonInterp 2.

Since VK-GL-CTS currently depends on Python 2, this patch performs
that check early, which will be reused by SPIRV-Tools (i.e. it will
accept python2 found in the system instead of possibly taking a
python3).

Component: Framework

Change-Id: I588c1a1cbe06b95e7fda5db6a1a9f9d046dbda90