nixos/go2rtc: add support for v4l2 video sources

Capturing from /dev/video* requires being member of the video group.

nixos/modules/services/video/go2rtc/default.nix

···
       serviceConfig = {
         DynamicUser = true;
         User = "go2rtc";
+        SupplementaryGroups = [
+          # for v4l2 devices
+          "video"
+        ];
         StateDirectory = "go2rtc";
         ExecStart = "${cfg.package}/bin/go2rtc -config ${configFile}";
       };