chore(shared/gnupg): add lib.mkDefault for programs.gnupg.agent.pinentryPackage
Follow-up to earlier commit on using pinentry-curses
by default per c38f768365169453907607c71f5e733262e6f6df, especially for desktop environment-specific overrides (e.g. KDE Plasma)
Signed-off-by: ajhalili2006 <ajhalili2006@gmail.com>