mktemp: document dry-run flag

[sudoAlphaX]

I wanted to use a similar feature where i wanted to create a temporary filename but not create the file yet. After a bit of searching, i realised mktemp had that built-in functionality. So, i thought it would be a good idea to add this as an example to the mktemp page.

Translations required.

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):
  mktemp (GNU coreutils) 9.7
  Copyright (C) 2025 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later https://gnu.org/licenses/gpl.html.
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.

Written by Jim Meyering and Eric Blake.