gnuplot: use lib.makeBinPath to condense wrapper

  # binary wrappers don't support --run

  postInstall = lib.optionalString withX ''

    wrapProgramShell $out/bin/gnuplot \

       --prefix PATH : '${gnused}/bin' \

       --prefix PATH : '${coreutils}/bin' \

       --prefix PATH : '${fontconfig.bin}/bin' \

       --prefix PATH : '${lib.makeBinPath [ gnused coreutils fontconfig.bin ]}' \

       "''${gappsWrapperArgs[@]}" \

       "''${qtWrapperArgs[@]}" \

       --run '. ${./set-gdfontpath-from-fontconfig.sh}'