Rename PXE installer menu entry, add vanilla NixOS minimal netboot entry

The iPXE menu's "nixos" entry actually chain-loads this flake's own
custom auto-installer image, not a stock NixOS image — rename it to
"auto-installer" (label "NixOS Auto-Installer") so the menu says what it
boots, and set networking.hostName on netbootSystem to match, so the
generated system name (nixos-system-auto-installer-*) and staged
directory (/srv/pxe/http/auto-installer) agree with the menu entry too.

Add a second, genuinely vanilla NixOS minimal netboot image
(netbootMinimalSystem in flake.nix — nixpkgs' netboot-minimal.nix on its
own, none of modules/installer/common.nix's auto-installer wiring),
built from source the same way as the auto-installer image and exposed
as packages.x86_64-linux.pxe-minimal. Staged and menu-wired the same
way, as "nixos-minimal" (item, hostname, and directory all matching).

modules/pxe-boot/stage-installer-artifacts.nix is generalized to stage
both images via a shared rule-builder instead of one hardcoded set of
paths.

Verified: nix eval confirms both images' config.system.name matches
their menu entry/directory names, the pxe-boot host itself builds
clean with the new menu.ipxe, and the new pxe-minimal image was booted
directly under QEMU (kernel+initrd, no KVM) to a working login shell
with hostname nixos-minimal, no hang.

modules/build-types/pxe-boot.nix

@@ -68,15 +68,19 @@ let
     set base ${pxeBaseUrl}
 
     menu PXE Boot Menu
-    item nixos NixOS Installer
+    item auto-installer NixOS Auto-Installer
+    item nixos-minimal NixOS Minimal
     item rescue Rescue Environment
     item shell iPXE Shell
     item reboot Reboot
 
     choose target && goto ''${target}
 
-    :nixos
-    chain ''${base}/nixos/netboot.ipxe
+    :auto-installer
+    chain ''${base}/auto-installer/netboot.ipxe
+
+    :nixos-minimal
+    chain ''${base}/nixos-minimal/netboot.ipxe
 
     :rescue
     chain ''${base}/systemrescue.ipxe
@@ -129,7 +133,8 @@ in
     "d ${pxeRoot} 0755 root root -"
     "d ${httpRoot} 0755 root root -"
     "d ${httpRoot}/images 0755 root root -"
-    "d ${httpRoot}/nixos 0755 root root -"
+    "d ${httpRoot}/auto-installer 0755 root root -"
+    "d ${httpRoot}/nixos-minimal 0755 root root -"
     "d ${httpRoot}/systemrescue 0755 root root -"
     "d ${httpRoot}/ubuntu 0755 root root -"
     "d ${httpRoot}/rescue 0755 root root -"