derivations: Restore UTF-8 encoding of build scripts.

Reported by Mark H Weaver <mhw@netris.org>
at <https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00457.html>.

* guix/derivations.scm (build-expression->derivation): Use a string
output port for the expression.  This reverts part of
2dce88d5bb.
* tests/derivations.scm ("build-expression->derivation and builder
encoding"): New test.

tests/derivations.scm

@@ -701,6 +701,19 @@
                                   #:modules '((guix module that
                                                     does not exist)))))
 
+(test-equal "build-expression->derivation and builder encoding"
+  '("UTF-8" #t)
+  (let* ((exp '(λ (α) (+ α 1)))
+         (drv (build-expression->derivation %store "foo" exp)))
+    (match (derivation-builder-arguments drv)
+      ((... builder)
+       (call-with-input-file builder
+         (lambda (port)
+           (list (port-encoding port)
+                 (->bool
+                  (string-contains (get-string-all port)
+                                   "(λ (α) (+ α 1))")))))))))
+
 (test-assert "build-expression->derivation and derivation-prerequisites"
   (let ((drv (build-expression->derivation %store "fail" #f)))
     (any (match-lambda