flameshot: provide meta.mainProgram

pkgs/tools/misc/flameshot/default.nix

···
   meta = with lib; {
     description = "Powerful yet simple to use screenshot software";
     homepage = "https://github.com/flameshot-org/flameshot";
+    mainProgram = "flameshot";
     maintainers = with maintainers; [ scode oxalica ];
     license = licenses.gpl3Plus;
     platforms = platforms.linux ++ platforms.darwin;