amazon-image: fetch metadata only in stage-2

This also removes automatic enablement/mounting of instance store swap
devices and ext3 filesystems. This behaviour is strongly opinionated
and shouldn't be enabled by default.

The unionfs behaviour never took effect anyway, because the AMI
manifest path only exists for instance store-backed AMIs, which have
not been supported by nixpkgs since
84742e2293 (2019).

nixos/modules/virtualisation/ec2-data.nix

@@ -18,6 +18,7 @@ with lib;
 
         wantedBy = [ "multi-user.target" "sshd.service" ];
         before = [ "sshd.service" ];
+        after = ["fetch-ec2-metadata.service"];
 
         path = [ pkgs.iproute2 ];