Fix failing Skopeo pullImage with `/run/containers` not readable

[norpol]

I've just debugged for someone why Skopeo fails with reading JSON file "/run/containers/1001/auth.json": open /run/containers/1001/auth.json: permission denied inside GitHub actions.

I think that is something also true for nix2container.pullImage, though I'm not sure how/where you would like me to add this change. Needs probably some further investigation.

To test this run mkdir /run/containers; chmod 000 /run/containers and see Skopeo pull failing.

https://github.com/NixOS/nixpkgs/pull/294849