pull: Install (guix config) module to override the user's one.

pull: Install (guix config) module to override the user's one.

* build-aux/build-self.scm (zlib, gzip, bzip2, xz): New variables.
(build)[storedir, localstatedir, sysconfdir, sbindir]: New variables.
[builder]: Pass them to 'build-guix'.
* guix/build/pull.scm (build-guix): Add #:system, #:storedir,
  #:localstatedir, #:sysconfdir, #:sbindir, #:package-name,
  #:package-version, #:bug-report-address, #:home-page-url, #:libgcrypt,
  #:zlib, #:gzip, #:bzip2, and #:xz.  Remove #:gcrypt.
  Instantiate all the substitution variables in (guix config).  Remove
  code to delete OUT/guix/config.{scm,go}.
* guix/config.scm.in: Add note about (guix script pull).