pull: Use the build procedure provided by the newly-downloaded Guix.

Fixes <http://bugs.gnu.org/18534>.

* guix/scripts/pull.scm (with-environment-variable, with-PATH): New
  macros.
  (temporary-directory, first-directory, interned-then-deleted): New
  procedures.
  (unpack): Rewrite to do the unpacking in the current process rather
  than as a separate derivation.
  (%self-build-file): New variable.
  (build-from-source): New procedure.
  (build-and-install): Use it.
* guix/build/pull.scm (build-guix): Rename 'tarball' argument to
  'source'.  Remove #:tar and #:gzip parameters, as well as 'tar'
  invocation.  Remove 'scandir' invocation.  Wrap body in
  'with-directory-excursion'.
* build-aux/build-self.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.