The placement of this option under `nix` was misleading, as it is not
configuration of the Nix daemon, but rather configuration of the NixOS
boot process and how it mounts the Nix store. As such, make it an option
of `boot` to clarify what it actually affects, and imply that it will
only take effect on a reboot.
Since it no longer has the context of nix, adjust the name to include
it.
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).
