Only use env vars, they were unused.

Use consistently a config object with properties instead of functions
(the one returning a binary are especially dangerous if parens are
forgotten).

Also add helpers to call the target python more succinctly.

We are mostly interested it installed alright.

To be used by the psycopg/psycopg2-wheels project.

It was performed before repos cloned so no resource available (including
the script!)