build-self: Accomodate upgrades from May 2016 and earlier.

Reported by Paul Garlick <pgarlick@tourbillion-technology.com>.

Fixes a bug whereby "old" Guix instances (before mid-May 2018) would
fail to pull to current master.

Specifically, Paul reported being unable to upgrade from
6f84dc43 (a commit that predates that
addition of the 'bootstrap' phase on gnu-build-system on 'master' and
that also predates the new 'guix pull'.)

* build-aux/build-self.scm (guile-gcrypt)[arguments]: New field.