fix: print the actual address listened on

This is useful when the server is started with an ephemeral port, e.g.:

./rest-server --no-auth --listen "127.0.0.1:0"
Data directory: /tmp/restic
Authentication disabled
Private repositories disabled
start server on 127.0.0.1:46015

cmd/rest-server/listener_unix.go

@@ -28,7 +28,7 @@ func findListener(addr string) (listener net.Listener, err error) {
 			return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
 		}
 
-		log.Printf("start server on %v", addr)
+		log.Printf("start server on %s", listener.Addr().String())
 		return listener, nil
 
 	case 1:

cmd/rest-server/listener_windows.go

@@ -14,6 +14,6 @@ func findListener(addr string) (listener net.Listener, err error) {
 		return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
 	}
 
-	log.Printf("start server on %v", addr)
+	log.Printf("start server on %s", listener.Addr().String())
 	return listener, nil
 }