add credentials

Author: parthea

.kokoro/presubmit/prerelease-deps.cfg

@@ -5,3 +5,9 @@ env_vars: {
     key: "NOX_SESSION"
     value: "prerelease_deps"
 }
+
+# Credentials needed to test universe domain.
+env_vars: {
+  key: "SECRET_MANAGER_KEYS"
+  value: "client-library-test-universe-domain-credential"
+}