spirv-tools: python3 is need not only for the tests

commit acf109c1a1a30907f7401f6225fcbfb5f2e39a58 remove the python3native
when disabling the tests but python is needed not only for the tests,
so re add it.

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>