visidata: add runtime dependency of clipboard commands

VisiData’s system clipboard commands—

  - Edit → Copy → to system clipboard → …
  - Edit → Paste → from system clipboard → …

—execute subprocesses defined by configuration options:

  - options.clipboard_copy_cmd
  - options.clipboard_paste_cmd

On Linux, these options default to using `xclip`:

  - https://github.com/saulpw/visidata/blob/v2.8/visidata/clipboard.py#L11-L22

Without it, the system clipboard commands fail with:

    FileNotFoundError: [Errno 2] No such file or directory: 'xclip'

pkgs/applications/misc/visidata/default.nix

@@ -22,6 +22,7 @@
 , setuptools
 , git
 , withPcap ? true, dpkt, dnslib
+, withXclip ? stdenv.isLinux, xclip
 }:
 buildPythonApplication rec {
  pname = "visidata";
@@ -63,7 +64,8 @@ buildPythonApplication rec {
  zstandard
  odfpy
  setuptools
- ] ++ lib.optionals withPcap [ dpkt dnslib ];
+ ] ++ lib.optionals withPcap [ dpkt dnslib ]
+ ++ lib.optional withXclip xclip;
 
  checkInputs = [
  git