chore: store artifacts in placer (#1821)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: chingor13

synthtool/gcp/templates/python_library/.kokoro/release/common.cfg

@@ -38,3 +38,12 @@ env_vars: {
   key: "SECRET_MANAGER_KEYS"
   value: "releasetool-publish-reporter-app,releasetool-publish-reporter-googleapis-installation,releasetool-publish-reporter-pem"
 }
+
+# Store the packages we uploaded to PyPI.  That way, we have a record of exactly
+# what we published, which we can use to generate SBOMs and attestations.
+action {
+  define_artifacts {
+    regex: "github/{{ metadata['repo']['repo'].split('/')[1] }}/**/*.tar.gz"
+    strip_prefix: "github/{{ metadata['repo']['repo'].split('/')[1] }}"
+  }
+}