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.
