nixos/installer: Refresh against upstream

The ISO target had drifted from nixpkgs and no longer evaluated:
`iso-image.nix` now defines `image.baseName` itself, which conflicts
with ours, so force it.

Drop the `boot.initrd.systemd.enable = false` override from the
`asISO` build target. The missing `/dev/root` it worked around is no
longer an issue, and scripted initrd is deprecated for removal in
26.11.

Replace the wpa_supplicant stanza, which upstream's
`installation-device.nix` no longer carries, with NetworkManager.
Keep it out of `multi-user.target` so nothing network-related starts
until asked; NetworkManager enables wpa_supplicant as its backend,
which is D-Bus activated on demand.

Pick up three more bits from that profile: the installer
`variant_id`, the pstore drop-in that stops an install evacuating the
target's persistent entries, and the mdadm `PROGRAM` stub that
silences the unset-mail warning.

Built and booted as an ISO to confirm.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/misc/installer.md

@@ -24,8 +24,11 @@ The custom NixOS installer image used to bootstrap new boxes.
   `installer-<hex>` hostname is set at boot.
 - `INSTALL_ROOT=/mnt` in the session environment, plus a `show-hw-config` alias wrapping
   `nixos-generate-config --show-hardware-config --root $INSTALL_ROOT`.
-- NixOS documentation enabled, `wpa_supplicant` available but not started, GC and memory-overcommit
-  tuning for low-memory targets, LVM thin and NFS support.
+- NixOS documentation enabled, NetworkManager available but not started at boot (run
+  `systemctl start NetworkManager`, then `nmtui`), GC and memory-overcommit tuning for low-memory
+  targets, LVM thin and NFS support.
+- Identifies itself as `VARIANT_ID=installer` in `/etc/os-release`, and leaves the target's
+  persistent pstore entries alone (`Unlink=no`) so an install doesn't evacuate them.
 - No regular user (`my.user.enable = false`), no tmpfs-root management, no NAT, and not a
   deploy target (`my.deploy.enable = false`).