self: Make (guix config) generation really stateless.

Previously the %CONFIG-VARIABLES list would be generated based on what
the current (guix config) contains.  Thus, it would include
'%guix-register-program', which we recently removed, because
existing (guix config) most likely contained that variable.  Since its
value could differ from machine to machine, the build farm could be
building a different config.scm, thereby preventing people from getting
substitutes.

* guix/self.scm (%config-variables): Turn into a white list instead of
taking all the remaining variables from the current (guix config).
* build-aux/build-self.scm (%config-variables): Likewise.