Fix missing default-policy.json in skopeo

We started having issues with `pkgs.dockerTools.pullImage`, were it
would fail with:

```
FATA[0000] Error loading trust policy: open /etc/containers/policy.json: no such file or directory
```

It turns out that since `skopeo` was bumped to `0.1.40`, it was
accidentally no longer being built with a default policy.

This may happen again, see https://github.com/containers/skopeo/issues/787