xps/15-7590: don't enable unfree firmware by default

Also don't set nixpkgs.pkgs. Instead just add an overlay.

dell/xps/15-7590/README.wiki

@@ -24,7 +24,9 @@ These settings are needed both for booting the final install, and installer itse
 === Wifi ===
 ~~Wifi does not work with kernels older than 5.1 (firmware not present) or newer~~ (https://bbs.archlinux.org/viewtopic.php?id=247705)
 
-Update: The 48.ucode causes the Killer wifi card to crash. The iwlfwifi-cc-a0-46.ucode works perfectly. default.nix contains an overlay that removes the offending ucode from the linux-firmware bundle.
+Update: The 48.ucode causes the Killer wifi card to crash. The iwlfwifi-cc-a0-46.ucode works perfectly. 
+default.nix contains an overlay that removes the offending ucode from the linux-firmware bundle.
+To use it one also needs to enable unfree firmware in their own configuration (<code>hardware.enableRedistributableFirmware = true;</code>)
 
 ```
   # Use the systemd-boot EFI boot loader.

dell/xps/15-7590/default.nix

@@ -18,22 +18,19 @@
 
   # The 48.ucode causes the Killer wifi card to crash.
   # The iwlfwifi-cc-a0-46.ucode works perfectly
-  nixpkgs.pkgs = import <nixpkgs> {
-    config.allowUnfree = true;
-    overlays = [
-      (self: super: {
-        firmwareLinuxNonfree = super.firmwareLinuxNonfree.overrideAttrs (old: {
-          src = super.fetchgit{
-            url = "https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git";
-            rev = "bf13a71b18af229b4c900b321ef1f8443028ded8";
-            sha256 = "1dcaqdqyffxiadx420pg20157wqidz0c0ca5mrgyfxgrbh6a4mdj";
-          };
-          postInstall = ''
-            rm $out/lib/firmware/iwlwifi-cc-a0-48.ucode
-          '';
-          outputHash = "0dq48i1cr8f0qx3nyq50l9w9915vhgpwmwiw3b4yhisbc3afyay4";
-        });
-      })
-    ];
-  };
+  nixpkgs.overlays = [
+    (self: super: {
+      firmwareLinuxNonfree = super.firmwareLinuxNonfree.overrideAttrs (old: {
+        src = super.fetchgit{
+          url = "https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git";
+          rev = "bf13a71b18af229b4c900b321ef1f8443028ded8";
+          sha256 = "1dcaqdqyffxiadx420pg20157wqidz0c0ca5mrgyfxgrbh6a4mdj";
+        };
+        postInstall = ''
+          rm $out/lib/firmware/iwlwifi-cc-a0-48.ucode
+        '';
+        outputHash = "0dq48i1cr8f0qx3nyq50l9w9915vhgpwmwiw3b4yhisbc3afyay4";
+      });
+    })
+  ];
 }