google: mark cache as ready after it got populated

The googleImagesCache is currently not marked `ready` AFAICT.
This means while it is correctly build it is not used in
subsequent calls to projectImages(). This commit set the
cache to ready after it got populated.

spread/google.go

@@ -369,6 +369,7 @@ func (p *googleProvider) projectImages(project string) ([]googleImage, error) {
 			Terms:   toTerms(item.Description),
 		})
 	}
+	cache.ready = true
 
 	return cache.images, err
 }