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.

tests/derivations.scm

@@ -21,6 +21,8 @@
   #:use-module (guix derivations)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module ((guix packages) #:select (package-derivation))
+  #:use-module ((distro packages base) #:select (%bootstrap-guile))
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
@@ -36,6 +38,11 @@
 (define %store
   (false-if-exception (open-connection)))
 
+(when %store
+  ;; By default, use %BOOTSTRAP-GUILE for the current system.
+  (let ((drv (package-derivation %store %bootstrap-guile)))
+    (%guile-for-build drv)))
+
 (define (directory-contents dir)
   "Return an alist representing the contents of DIR."
   (define prefix-len (string-length dir))