build-system/asdf: 'package-with-build-system' accesses inputs lazily.

build-system/asdf: 'package-with-build-system' accesses inputs lazily.

Fixes a bug whereby we would, at the top-level (while evaluation
lisp.scm package definitions), attempt to access package inputs.
Because of circular dependencies, this could lead to unbound variables
as reported by Arun Isaac <arunisaac@systemreboot.net> at
<https://debbugs.gnu.org/cgi/bugreport.cgi?bug=33801#56>.

* guix/build-system/asdf.scm (package-with-build-system)[transform]
(new-propagated-inputs): Turn into a procedure.
Adjust user accordingly.