ssm-agent expects files in /etc/amazon/ssm. The pkg substitutes a location in
the nix store for those default files, but if we ever want to adjust this
configuration on NixOS, we'd need the ability to modify that file.
This change to the nixos module writes copies of the default files from the nix
store to /etc/amazon/ssm. Future versions can add config, but right now this
would allow users to at least write out a text value to
environment.etc."amazon/ssm/amazon-ssm-agent.json".text to provide
their own config.
