feat: Fallback to nixpkgs#nixos-facter if not available in booted system

[a-kenji]

Fallback to nixos-facter from nixpkgs if not available in the booted system.
This way we can skip the specific image requirement, while still
supporting the specific nixos-facter in a booted image, if
available.
Can improve bootstrapping of nixos-anywhere.

Alternatives:

• Since this implementation directly runs nixos-facter from nixpkgs
  we could theoretically get an uncached version and would need to
  build. We could test for that with --max-jobs 0 and give an
  approriate error message.

Tested:

• This has been tested manually, but if required I can add a nixosTest for the functionality.

[Enzime]

Looks like nixOptions isn't being quoted correctly over SSH:

If I run this on a machine without experimental-features = nix-command flakes:

+ step 'Generating facter.json using nixos-facter from nixpkgs'
+ echo '### Generating facter.json using nixos-facter from nixpkgs ###'
+ runSshNoTty -o ConnectTimeout=10 '' nix run --extra-experimental-features 'nix-command flakes' --no-write-lock-file -L nixpkgs#nixos-facter
+ ssh -o IdentitiesOnly=yes -i /tmp/tmp.VQpXObgWld/nixos-anywhere -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no root@example.local -o ConnectTimeout=10 '' nix run --extra-experimental-features 'nix-command flakes' --no-write-lock-file -L nixpkgs#nixos-facter
Warning: Permanently added 'example.local' (ED25519) to the list of known hosts.
error: experimental Nix feature 'flakes' is disabled; add '--extra-experimental-features flakes' to enable it

If I run this on a machine with experimental-features = nix-command flakes:

+ step 'Generating facter.json using nixos-facter from nixpkgs'
+ echo '### Generating facter.json using nixos-facter from nixpkgs ###'
+ runSshNoTty -o ConnectTimeout=10 '' nix run --extra-experimental-features 'nix-command flakes' --no-write-lock-file -L nixpkgs#nixos-facter
+ ssh -o IdentitiesOnly=yes -i /tmp/tmp.xak2g9IHuf/nixos-anywhere -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no root@example.local -o ConnectTimeout=10 '' nix run --extra-experimental-features 'nix-command flakes' --no-write-lock-file -L nixpkgs#nixos-facter
Warning: Permanently added 'example.local' (ED25519) to the list of known hosts.
error: cannot find flake 'flake:flakes' in the flake registries