chore: Declare gem package artifacts in release job (#159)

Author: dazuma

.kokoro/release.cfg

@@ -31,3 +31,12 @@ env_vars: {
   key: "SECRET_MANAGER_KEYS"
   value: "releasetool-publish-reporter-app,releasetool-publish-reporter-googlecloudplatform-installation,releasetool-publish-reporter-pem,docuploader_service_account"
 }
+
+# Store the packages uploaded to rubygems.org, which
+# we can later use to generate SBOMs and attestations.
+action {
+ define_artifacts {
+   regex: "github/functions-framework-ruby/pkg/*.gem"
+   strip_prefix: "github"
+ }
+}