secretsmgr: actually _enable_ the timer unit

ops/nixos/lib/secretsmgr.nix

@@ -123,6 +123,7 @@ in
     systemd.timers.secretsmgr = {
       requires = ["vault-agent.service"];
       after = ["vault-agent.service" "network-online.target"];
+      wantedBy = ["timers.target"];
 
       timerConfig = {
         OnActiveSec = "30";