curl does not retry if it is unable to connect to the metadata server.
For some reason, when creating a new AMI with a recent nixpkgs, the
metadata server would not be available when fetch-ec2-data ran. Switching
to wget that can retry even on TCP connection errors solved this problem.
I also made the fetch-ec2-data depend on ip-up.target, to get it to start
a bit later.
534a01c2b0