Merge pull request #250668 from arcnmx/xss-mainProgram

pkgs/misc/screensavers/xss-lock/default.nix

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   meta = with lib; {
     description = "Use external locker (such as i3lock) as X screen saver";
     license = licenses.mit;
+    mainProgram = "xss-lock";
     maintainers = with maintainers; [ malyn offline ];
     platforms = platforms.linux;
   };