We used to publish test artifacts to a Maven repo on GCS, for use by
schema tests. For this to work with Kokoro, the GCS bucket must be
accessible to all users.
To comply with the no-public-user requirement, we store the necessary
jars at at well-known bucket and map them into Kokoro. This strategy
cannot be used on the Maven repo because only a small number of files
with fixed names may be mapped. With the Maven repo, there are too many
files to map.