Build agent image in cloud build (#724)

Previously, cloud build pulls an agent image that I manually uploaded to
artifact registry.
However, this will fail when we update dependencies (e.g., upgrade
python packages) in PRs.

This PR builds the image with the PR source code so that its agent image
is always up-to-date.

common/cloud_builder.py

@@ -132,7 +132,16 @@ def _request_cloud_build(self, ofg_repo_url: str, agent_dill_url: str,
                 'dir': '/workspace',
                 'args': ['clone', '--depth=1', OF_REPO, 'ofg/oss-fuzz']
             },
-            # Step 3: Run the Python script with the dill files
+            # Step 3: Run the Python script with the dill files.
+            {
+                'name': 'gcr.io/cloud-builders/docker',
+                'args': [
+                    'build', '.', '-t',
+                    ('us-central1-docker.pkg.dev/oss-fuzz/oss-fuzz-gen/'
+                     'agent-image'), '-f', 'Dockerfile.cloudbuild-agent'
+                ],
+                'dir': '/workspace/ofg/',
+            },
             {
                 'id':
                     'agent-step',