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

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.

Author: a-kenji

src/nixos-anywhere.sh

@@ -582,12 +582,17 @@ generateHardwareConfig() {
   case "$hardwareConfigBackend" in
   nixos-facter)
     if [[ ${isInstaller} == "y" ]]; then
-      if [[ ${hasNixOSFacter} == "n" ]]; then
-        abort "nixos-facter is not available in booted installer, use nixos-generate-config. For nixos-facter, you may want to boot an installer image from here instead: https://github.com/nix-community/nixos-images"
-      fi
-    else
       maybeSudo=""
     fi
+    if [[ ${hasNixOSFacter} == "n" ]]; then
+      step "Generating hardware-configuration.nix using nixos-facter from nixpkgs"
+      runSshNoTty -o ConnectTimeout=10 "${maybeSudo}" \
+        nix run "${nixOptions[@]}" \
+        nixpkgs#nixos-facter >"$hardwareConfigPath"
+    else
+      step "Generating hardware-configuration.nix using nixos-facter"
+      runSshNoTty -o ConnectTimeout=10 "${maybeSudo}" "nixos-facter" >"$hardwareConfigPath"
+    fi
 
     step "Generating hardware-configuration.nix using nixos-facter"
     runSshNoTty -o ConnectTimeout=10 ${maybeSudo} "nixos-facter" >"$hardwareConfigPath"