derivations: Pass the derivation of guile-for-build to `imported-files' & co.

derivations: Pass the derivation of guile-for-build to `imported-files' & co.

* 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.