cmd/derper: close setec after use (#14929)

Since dynamic reload of setec is not supported
in derper at this time, close the server after
the secret is loaded.

Updates tailscale/corp#25756

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

cmd/derper/derper.go

@@ -224,6 +224,7 @@ func main() {
 		}
 		meshKey = st.Secret(meshKeySecret).GetString()
 		log.Println("Got mesh key from setec store")
+		st.Close()
 	} else if *meshPSKFile != "" {
 		b, err := setec.StaticFile(*meshPSKFile)
 		if err != nil {