Pin avr-gcc in shell.nix pending release of 8.3.0 (#3922)

* Pin avr-gcc in shell.nix pending release of 8.3.0

There's apparently a critical bug in 8.2.0, which is now the nixpkgs default. This change overrides that default in favor of the known good version. Once 8.3.0 is the default, the override can be dropped.

* Arch/Manjaro fix

shell.nix

@@ -13,6 +13,13 @@ let
     "-B${avrlibc}/avr/lib/avr51"
     "-L${avrlibc}/avr/lib/avr51"
   ];
+  avrgcc = pkgs.avrgcc.overrideAttrs (oldAttrs: rec {
+    name = "avr-gcc-8.1.0";
+    src = fetchurl {
+      url = "mirror://gcc/releases/gcc-8.1.0/gcc-8.1.0.tar.xz";
+      sha256 = "0lxil8x0jjx7zbf90cy1rli650akaa6hpk8wk8s62vk2jbwnc60x";
+    };
+  });
 in
 
 stdenv.mkDerivation {