open-policy-agent: skip test too dependent on go version (#428831)

pkgs/by-name/op/open-policy-agent/package.nix

···
         "TestInterQueryCache_ClientError"
         "TestIntraQueryCache_ClientError"
         "TestSSOCredentialService"
+
+        # This test depends on the metrics available in go not changing. This is a bit
+        # too unstable for us updating go independently.
+        "TestJSONSerialization"
       ]
       ++ lib.optionals stdenv.hostPlatform.isDarwin [
         # Skip tests that require network, not available in the darwin sandbox