Fix and re-enable imagegen_test.go (1/2: Fix.)

PIL requires a string format name, whereas `Format.PNG` is an "enum string"
which is not quite the same type.

I had not noticed this problem because I had manually tested the test on a VM
where the cached Stable Diffusion XL Docker image was from before this change.

Tested on a fresh VM with a freshly-built Docker image now.

This is split in two changes because the image needs to be rebuilt.
The first change updates the image, and the second change re-enables the test.

PiperOrigin-RevId: 618048266

images/gpu/stable-diffusion-xl/generate_image.py

@@ -241,7 +241,7 @@ if args.format in (Format.PNG, Format.JPEG):
   image.save(args.out, args.format)
 else:
   buf = io.BytesIO()
-  image.save(buf, format=Format.PNG)
+  image.save(buf, format=Format.PNG.value)
   image_bytes = buf.getvalue()
   time_done = datetime.datetime.now(datetime.timezone.utc)
   with open(args.out, 'wb') as f: