gexp: Leave grafting as is when lowering allowed/disallowed references.

Fixes <https://issues.guix.gnu.org/50676>.
Reported by Mathieu Othacehe <othacehe@gnu.org>.

Commit a779363b6a was partially incorrect:
references passed to #:allowed-references or #:references-graphs *can*
be lowered as references to grafted elements.  This is for example the
case when doing:

  (computed-file "partition.img" exp
                  #:options `(#:references-graphs ,inputs))

Here INPUTS must be lowered as a reference to suitably grafted elements.
Failing to do that, the reference graph will not match the actual
INPUTS.

However, when building a package, those references must indeed refer
only to ungrafted packages.  This commit preserves that by having build
systems pass #:graft? #f.

* guix/gexp.scm (lower-reference-graphs, lower-references): Remove uses
of 'without-grafting'.  This reverts
a779363b6a.
* guix/build-system/cmake.scm (cmake-build, cmake-cross-build):
Pass #:graft? #f.
* guix/build-system/glib-or-gtk.scm (glib-or-gtk-build)
(glib-or-gtk-cross-build): Likewise.
* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Likewise.
* guix/build-system/meson.scm (meson-build, meson-cross-build): Likewise.
* guix/build-system/trivial.scm (trivial-build, trivial-cross-build):
Likewise.
* tests/gexp.scm ("lower-object, computed-file + grafts"): New test.
* tests/packages.scm ("trivial with #:allowed-references + grafts"): New
test.

tests/packages.scm

@@ -882,6 +882,28 @@
       (build-derivations %store (list d))
       #f)))
 
+(test-assert "trivial with #:allowed-references + grafts"
+  (let* ((g (package
+              (inherit %bootstrap-guile)
+              (replacement (package
+                             (inherit %bootstrap-guile)
+                             (version "9.9")))))
+         (p (package
+              (inherit (dummy-package "trivial"))
+              (build-system trivial-build-system)
+              (inputs (list g))
+              (arguments
+               `(#:guile ,g
+                 #:allowed-references (,g)
+                 #:builder (mkdir %output)))))
+         (d0 (package-derivation %store p #:graft? #f))
+         (d1 (parameterize ((%graft? #t))
+               (package-derivation %store p #:graft? #t))))
+    ;; D1 should be equal to D2 because there's nothing to graft.  In
+    ;; particular, its #:disallowed-references should be lowered in the same
+    ;; way (ungrafted) whether or not #:graft? is true.
+    (string=? (derivation-file-name d1) (derivation-file-name d0))))
+
 (test-assert "search paths"
   (let* ((p (make-prompt-tag "return-search-paths"))
          (t (make-parameter "guile-0"))