Stop depending on GCS public access for Kokoro (#2907)

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.

gradle.properties

@@ -22,9 +22,8 @@ dbServer=
 dbName=postgres
 dbUser=
 dbPassword=
-publish_repo=
-baseSchemaTag=
-schema_version=
-nomulus_version=
 dot_path=/usr/bin/dot
 pipeline=
+nomulus_env=
+schema_env=
+schemaTestArtifactsDir=