We can not rely on system python here, it's a bit better to rely on
buildout's python ( the main reason is that it complies with
slapos-sr-testing not having system python ).

The implementation is a bit complicated, we can not just reference
${buildout:executable} in the profile, because doing so will cause an
infinite loop with rebootstrap and software installation never finish
because options of ca-certificates change at every run if they include
the python path. Instead, we use a pre-make-hook to rewrite the Makefile
without saving the interpreter in option. This python is only used
during the build process (to generate the certificate files) and not
used at run time, so it does not really depend on python strictly
speaking. Anyway, because it's a component used very early in the
bootstrap, we can not reference python here.