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 containers/skopeo#787

pkgs/development/tools/skopeo/default.nix

@@ -33,8 +33,8 @@ buildGoPackage {
 
   buildFlagsArray = ''
     -ldflags=
-    -X github.com/containers/skopeo/vendor/github.com/containers/image/signature.systemDefaultPolicyPath=${defaultPolicyFile}
-    -X github.com/containers/skopeo/vendor/github.com/containers/image/internal/tmpdir.unixTempDirForBigFiles=/tmp
+    -X github.com/containers/skopeo/vendor/github.com/containers/image/v5/signature.systemDefaultPolicyPath=${defaultPolicyFile}
+    -X github.com/containers/skopeo/vendor/github.com/containers/image/v5/internal/tmpdir.unixTempDirForBigFiles=/tmp
   '';
 
   preBuild = ''