unshare-net fails on gvisor due to failing loopback setup

I am running bwrap in a docker that runs on GCP, thus under gvisor.

When I try to use `--unshare-all` or `--unshare-net`, the bwrap code fails in:
```
if (opt_unshare_net)
loopback_setup (); /* Will exit if unsuccessful */
```

Is there a reason to try to setup loopback?

I have removed this code part and can confirm that then bwrap works on gvisor, too.

Thanks for any suggestion