system: Accept gexps in 'setuid-programs'.

Commit a7ac19851b led configs such as the
following one, which were previously valid, to be rejected:

  (operating-system
    ;; ...
    (setuid-programs (cons #~(string-append #$wireshark "/bin/dumpcap")
                           %setuid-programs)))

They are now accepted again.

Reported by wonko on #guix.

* gnu/system.scm (%ensure-setuid-program-list): Handle the case where
PROGRAM is not a file-like.

gnu/system.scm

@@ -1082,11 +1082,13 @@ (define (warn-once)
       (set! warned? #t)))
 
   (map (match-lambda
-         ((? file-like? program)
-          (warn-once)
-          (setuid-program (program program)))
          ((? setuid-program? program)
-          program))
+          program)
+         (program
+          ;; PROGRAM is a file-like or a gexp like #~(string-append #$foo
+          ;; "/bin/bar").
+          (warn-once)
+          (setuid-program (program program))))
        lst))
 
 (define %setuid-programs