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>

nixos/modules/build.nix

@@ -18,9 +18,6 @@ let
       "${modulesPath}/installer/cd-dvd/iso-image.nix"
       allHardware
       {
-        # Doesn't work right now... (missing /dev/root)
-        boot.initrd.systemd.enable = false;
-
         isoImage = {
           makeEfiBootable = true;
           makeUsbBootable = true;