* guix/derivations.scm (%guile-for-build): Initialize to #f.
(imported-files, imported-modules, compiled-modules): Add `guile'
keyword parameter. Pass it down to `build-expression->derivation'.
(build-expression->derivation)[guile-drv]: New variable. Pass it as
the #:guile parameter for `imported-modules' and `compiled-modules'.
* tests/derivations.scm: Set %GUILE-FOR-BUILD to the derivation of
%BOOTSTRAP-GUILE.