vbecome: Unset V23_CREDENTIALS

Ivan pointed out that vbecome behaves incorrectly if you have V23_CREDENTIALS
set.
Long-term: Matt says we should work to remove V23_CREDENTIALS completely.

For now we will just make vbecome unset V23_CREDENTIALS.

This resolves https://github.com/vanadium/issues/issues/729.

Change-Id: I3d83dcedd277263cd29d5c1cd38982370876ae1b
diff --git a/services/agent/vbecome/vbecome.go b/services/agent/vbecome/vbecome.go
index 15abcc0..1b91eb2 100644
--- a/services/agent/vbecome/vbecome.go
+++ b/services/agent/vbecome/vbecome.go
@@ -109,6 +109,11 @@
 		}
 	}
 
+	// Clear out the environment variable before starting the child.
+	if err = ref.EnvClearCredentials(); err != nil {
+		return err
+	}
+
 	// Start an agent server.
 	i := ipc.NewIPC()
 	if err := server.ServeAgent(i, principal); err != nil {