fix(infra): Correct variable name in trial builds

The variable `BASE_IMAGES` was renamed to `BASE_IMAGE_DEFS` in the
`base_images` module, but this change was not propagated to the trial
build script, causing an `AttributeError`.

This commit updates `build_and_push_test_images.py` to use the correct
`BASE_IMAGE_DEFS` variable, resolving the error and allowing trial
builds to run successfully.

Author: hunsche

infra/build/functions/build_and_push_test_images.py

@@ -217,9 +217,10 @@ def gcb_build_and_push_images(test_image_tag: str, version_tag: str = None):
   test_image_names = []
   versions = [version_tag] if version_tag else BASE_IMAGE_VERSIONS
   for version in versions:
-    for base_image in base_images.BASE_IMAGES:
-      main_image_name, test_image_name = get_image_tags(base_image.name,
-                                                        test_image_tag, version)
+    for base_image_def in base_images.BASE_IMAGE_DEFS:
+      base_image = base_images.ImageConfig(version=version, **base_image_def)
+      main_image_name, test_image_name = get_image_tags(
+          base_image.name, test_image_tag, version)
       test_image_names.append(test_image_name)
 
       if version == 'legacy':