Previously we'd pass a relative file name to 'add-indirect-root', which
the daemon would interpret as relative to "/". Consequently, checkouts
were not protected from GC.
* build-aux/update-guix-package.scm (main): Pass an absolute file name
to 'add-indirect-root'.
* build-aux/update-guix-package.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
(update-guix-package): New target.
(.PHONY): Add it.
* gnu/packages/package-management.scm (guix): Mention it.