Do not relocate /nix and /tmp to small disks on AWS

The default behavior with an m3.medium instance is to relocate
/nix and /tmp to /disk0 because an assumption is made that any
ephemeral disk is larger than the root volume.  Rather than make
that assumption, add a check to see if the disk is larger, and
only then relocate /nix and /tmp.

This addresses https://github.com/NixOS/nixpkgs/issues/12613