pack: Add UTF-8 locales to RPM pack builder.

It's necessary as 'generate-header' call ends up calling 'find-files', which
could fail to read file names containing non-ascii characters, as spotted in
the wild:

   building /gnu/store/...-jami-rpm-pack.rpm.drv...
   find-files: ./gnu/store/...-nss-certs-3.81/etc/ssl/certs/NetLock_Arany_=Class_Gold=_F??tan??s??tv??ny.pem:
   No such file or directory

* guix/scripts/pack.scm (rpm-archive): Expand set-utf8-locale helper in the
builder gexp.

guix/scripts/pack.scm

@@ -945,6 +945,9 @@ PREUN-FILE and POSTUN-FILE can be provided via EXTRA-OPTIONS."
                          (rnrs bytevectors)
                          (srfi srfi-1))
 
+            ;; Make sure non-ASCII file names are properly handled.
+            #+(set-utf8-locale profile)
+
             (define machine-type
               (and=> (or #$target %host-type)
                      (lambda (triplet)