grafts: Run with a UTF-8 locale.

Fixes <https://issues.guix.gnu.org/55968>.
Reported by Maxime Devos <maximedevos@telenet.be>.

* guix/grafts.scm (%graft-with-utf8-locale?): New parameter.
(graft-derivation/shallow)[glibc-locales, set-utf8-locale]: New
variables.
[build]: Use 'set-utf8-locale'.
* tests/gexp.scm, tests/grafts.scm, tests/packages.scm: Set
'%graft-with-utf8-locale?' to #f.

tests/gexp.scm

@@ -21,6 +21,7 @@
   #:use-module (guix store)
   #:use-module (guix monads)
   #:use-module (guix gexp)
+  #:use-module ((guix grafts) #:select (%graft-with-utf8-locale?))
   #:use-module (guix derivations)
   #:use-module (guix packages)
   #:use-module (guix build-system trivial)
@@ -49,6 +50,9 @@
 ;; Globally disable grafts because they can trigger early builds.
 (%graft? #f)
 
+;; When grafting, do not add dependency on 'glibc-utf8-locales'.
+(%graft-with-utf8-locale? #f)
+
 ;; For white-box testing.
 (define (gexp-inputs x)
   ((@@ (guix gexp) gexp-inputs) x))