cudatoolkit: Fix GUI applications

This fixes the following error when starting nvvp or nsight:

```
(java:23876): Gtk-WARNING **: 19:12:17.777: Unable to locate theme engine in module_path: "adwaita",

A fatal error has been detected by the Java Runtime Environment:

 SIGSEGV (0xb) at pc=0x00007f957f7b67fe, pid=23876, tid=140281059333888

JRE version: Java(TM) SE Runtime Environment (8.0_77-b03) (build 1.8.0_77-b03)
Java VM: Java HotSpot(TM) 64-Bit Server VM (25.77-b03 mixed mode linux-amd64 compressed oops)
Problematic frame:
C  [libpixbufloader-bmp.so+0x27fe]  gdk_pixbuf__bmp_image_load_increment+0xeee
```

Since GDK_PIXBUF_MODULE_FILE should match the version we actually link
to, we override the environment variable using the one provided by the
gdk_pixbuf setup-hook.