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

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: 618874414

test/gpu/imagegen_test.go

@@ -25,7 +25,6 @@ import (
 
 // TestStableDiffusionXL generates an image with Stable Diffusion XL.
 func TestStableDiffusionXL(t *testing.T) {
-	t.Skip("TODO(b/306706609): Fix this test.")
 	ctx := context.Background()
 	sdxl := stablediffusion.NewDockerXL(t)
 	generateCtx, generateCancel := context.WithTimeout(ctx, 15*time.Minute)