Making openimageio use parallel building.

(cherry picked from commit ff31b14cc51d3f493c62fb3074edc0c57fe8b7ce)

pkgs/applications/graphics/openimageio/default.nix

···
     "-DUSE_PYTHON=OFF"
   ];
 
-  buildPhase = ''
-    make ILMBASE_HOME=${ilmbase} OPENEXR_HOME=${openexr} USE_PYTHON=0 \
-      INSTALLDIR=$out dist_dir=
+  preBuild = ''
+    makeFlags="ILMBASE_HOME=${ilmbase} OPENEXR_HOME=${openexr} USE_PYTHON=0
+      INSTALLDIR=$out dist_dir="
   '';
 
   enableParallelBuilding = true;