Prevent unbootable systems if LVM snapshots are present at boot time

[brianmcgee]

For reference https://github.com/NixOS/nixpkgs/blob/dac9cdf8c930c0af98a63cbfe8005546ba0125fb/nixos/modules/installer/tools/nixos-generate-config.pl#L265-L268

TODO

• Detect if LVM snapshots are being used nixos-facter#5

[Mic92]

For systems that use disko it's actually better if we do not detect this in facter and instead make this part of disko: nix-community/disko#796

[Mic92]

Reason is that lvm won't be created at the time we run nixos-facter (we want facter before disko)

[brianmcgee]

This would fall under an "ephemeral" system property and live behind the --ephem flag, meaning by default it wouldn't be captured.

[Mic92]

We need it later for the non-disko use case.

[Mic92]

For the time being we should mention it in documentation for non-disko users.