Reported by Paul Garlick <email@example.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 fromgn-latest-20200428
6f84dc4314(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.