Fixed packaging for the "stream" application.

pkgs/stream/default.nix

@@ -9,7 +9,7 @@ stdenv.mkDerivation {
   nativeBuildInputs = [ gfortran ];
 
   installPhase = ''
-    mkdir $out/bin/
+    mkdir -p $out/bin/
     cp stream_c.exe $out/bin
     cp stream_f.exe $out/bin
   '';