nixos/modules/profiles/base.nix: omit zfs if unavailable
The `boot.zfs.enabled` option is marked `readOnly`, so this is the only way to
successfully build a NixOS installer image for platforms that zfs does not build
for.
Co-authored-by: Alyssa Ross <hi@alyssa.is>
authored by Adam Joseph Alyssa Ross and committed by Alyssa Ross 4cdda329 fe206915