Using swap for install

[pmaterer]

I'm using nixos-anywhere to install NixOS on a machine with 1GB of memory. During the Disko phase the /nix/store runs out of space (tmpfs filesystem running in memory). So I've taken to using a larger (30GB) external disk for swap. And this works, but when it gets to the reboot phase it fails. This is because I'm configuring zpools using Disko, and nixos-anywhere runs swapoff -a if it detects zpools. I understand this was added because of #420 (@TilCreator is using swap on a zpool) which makes sense... but maybe we can add a flag to disable this behavior. This flag would be useful when you are using swap for the install and not using swap on a zpool.

[pmaterer]

For clarity, I immediately get an "Out of memory" error after swap is turned off and my zpools do not get exported.

[pmaterer]

Some ideas:

• My initial idea was to add a new nixos-anywhere flag (something like --no-swapoff). If this flag is set simply skip the swapoff -a step.
• Somehow check if the swap devices that are on are using ZFS and only turn those off.
• Make users label their ZFS swap devices. Then require a flag, something like --swapoff-labels. Only turn those swap devices off.

Any thoughts?