update shell.nix (qmk#8910)

* now uses gcc 8.4
* fixes building boards with adafruit feather (in my case pancake)

shell.nix

@@ -8,26 +8,21 @@ let
     in {
       dfu-programmer = addDarwinSupport super.dfu-programmer;
       teensy-loader-cli = addDarwinSupport super.teensy-loader-cli;
-
-      avrgcc = super.avrgcc.overrideAttrs (oldAttrs: rec {
-        name = "avr-gcc-8.1.0";
-        src = super.fetchurl {
-          url = "mirror://gcc/releases/gcc-8.1.0/gcc-8.1.0.tar.xz";
-          sha256 = "0lxil8x0jjx7zbf90cy1rli650akaa6hpk8wk8s62vk2jbwnc60x";
-        };
-      });
     };
 
   nixpkgs = builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/0260747427737b980f0.tar.gz";
-    sha256 = "1p2yc6b40xvvxvmlqd9wb440pkrimnlc2wsbpa5rddlpx1dn8qmf";
+    url = "https://github.com/NixOS/nixpkgs/archive/903266491b7b9b0379e88709feca0af900def0d9.tar.gz";
+    sha256 = "1b5wjrfgyha6s15k1yjyx41hvrpmd5szpkpkxk6l5hyrfqsr8wip";
   };
 
   pkgs = import nixpkgs { overlays = [ overlay ]; };
 in
 
 with pkgs;
-let avr_incflags = [
+let 
+  avrlibc = pkgsCross.avr.libcCross;
+
+  avr_incflags = [
     "-isystem ${avrlibc}/avr/include"
     "-B${avrlibc}/avr/lib/avr5"
     "-L${avrlibc}/avr/lib/avr5"
@@ -41,7 +36,12 @@ stdenv.mkDerivation {
   name = "qmk-firmware";
 
   buildInputs = [ dfu-programmer dfu-util diffutils git python3 ]
-    ++ lib.optional avr [ avrbinutils avrgcc avrlibc avrdude ]
+    ++ lib.optional avr [ 
+      pkgsCross.avr.buildPackages.binutils
+      pkgsCross.avr.buildPackages.gcc8
+      avrlibc
+      avrdude
+    ]
     ++ lib.optional arm [ gcc-arm-embedded ]
     ++ lib.optional teensy [ teensy-loader-cli ];