gretl: disable install parallelism

Without the change parallel installs fail as:

    cp: cannot stat '...-gretl-2022c/share/gretl/data/plotbars': Not a directory
    make[1]: *** [Makefile:73: install_datafiles] Error 1

pkgs/applications/science/math/gretl/default.nix

@@ -28,6 +28,10 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ pkg-config ];
 
   enableParallelBuilding = true;
+  # Missing install depends:
+  #  cp: cannot stat '...-gretl-2022c/share/gretl/data/plotbars': Not a directory
+  #  make[1]: *** [Makefile:73: install_datafiles] Error 1
+  enableParallelInstalling = false;
 
   meta = with lib; {
     description = "A software package for econometric analysis";