bug#34216: Shroud require xclip for x clipboard

[nly <nly@disroot.org>]

> /gnu/store/w862nkwzvv2m3ng5lg5zlic1i962g416-gnupg-2.2.12           238.8    10.1   3.8%
> /gnu/store/agz9m74v7235h30mc3bdmj2nmyxqga4c-xclip-0.13             102.7     0.1   0.0%
> Looks like xclip is already in the closure of shroud,

I see, thanks.

> patch the reference to it so shroud can always find xclip.

How? 

Snippet from (shroud utils):

;; Instance of use of xclip
(define (call-with-clipboard proc)
  "Call PROC with an open output port to the X clipboard."
  (call-with-output-pipe* (list %xclip "-selection" "clipboard") proc))

;; Use of gpg
(define (call-with-encrypted-output-file file user-id proc)
  "Apply PROC with an output port that writes encrypted data to FILE
for the recipient USER-ID."
  (call-with-output-pipe* `(,(gpg-binary)
                            "--no-tty" "--batch" "--yes"
                            "--encrypt" "--armor"
                            "--recipient" ,user-id
                            "--output" ,file)
    proc))

Thanks,
amar