Fix building with empty NIX_PATH

<nixpkgs> lookup was being used in all hardware-configuration.nix files.
Turns out nixos-generate-config was fixed to not rely on <nixpkgs>
lookup in master branch, ref c45295d47e91fef3fdb13ee11fc24ebff8ad7288
("nixos/nixos-generate-config.pl: use modulesPath instead of <nixpkgs>").
This change manually fixes the hardware-configuration.nix files to match
that change, allowing `NIX_PATH= nix-build` to succeed.

Author: bjornfor

machines/media/hardware-configuration.nix

@@ -1,11 +1,11 @@
 # Do not modify this file!  It was generated by ‘nixos-generate-config’
 # and may be overwritten by future invocations.  Please make changes
 # to /etc/nixos/configuration.nix instead.
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, modulesPath, ... }:
 
 {
   imports =
-    [ <nixpkgs/nixos/modules/installer/scan/not-detected.nix>
+    [ (modulesPath + "/installer/scan/not-detected.nix")
     ];
 
   boot.initrd.availableKernelModules = [ "xhci_pci" "ehci_pci" "ahci" "usbhid" "usb_storage" "sd_mod" "sr_mod" "bcache" ];

machines/mini/hardware-configuration.nix

@@ -1,10 +1,10 @@
 # This is a generated file.  Do not modify!
 # Make changes to /etc/nixos/configuration.nix instead.
-{ config, pkgs, ... }:
+{ config, pkgs, modulesPath, ... }:
 
 {
   require = [
-    <nixpkgs/nixos/modules/installer/scan/not-detected.nix>
+    (modulesPath + "/installer/scan/not-detected.nix")
   ];
 
   boot.initrd.kernelModules = [ "xhci_hcd" "ehci_pci" "ahci" "usbhid" "usb_storage" ];

machines/srv1/hardware-configuration.nix

@@ -1,11 +1,11 @@
 # Do not modify this file!  It was generated by ‘nixos-generate-config’
 # and may be overwritten by future invocations.  Please make changes
 # to /etc/nixos/configuration.nix instead.
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, modulesPath, ... }:
 
 {
   imports =
-    [ <nixpkgs/nixos/modules/installer/scan/not-detected.nix>
+    [ (modulesPath + "/installer/scan/not-detected.nix")
     ];
 
   boot.initrd.availableKernelModules = [ "ahci" "xhci_pci" "ehci_pci" "nvme" "usbhid" "usb_storage" "sd_mod" "bcache" ];