Pin to an older nixpkgs with a gcc-arm-embedded that builds

To build with a newer gcc requires fixing `-Werror=array-bounds` issues in the bootloader.

nix/pinned-nixpkgs.json

@@ -0,0 +1,4 @@
+{
+  "url": "https://releases.nixos.org/nixpkgs/22.05-darwin/nixpkgs-darwin-22.05pre383202.058d97782ed/nixexprs.tar.xz",
+  "sha256": "0m00195y7l2b3mrzi26ljm32s24qp5paz15fqbd49iyfjzxv7xb8"
+}

nix/pinned-nixpkgs.nix

@@ -0,0 +1,17 @@
+{ system ? builtins.currentSystem }:
+
+let
+  pin = builtins.fromJSON (builtins.readFile ./pinned-nixpkgs.json);
+
+  nixpkgsSrc = builtins.fetchTarball {
+     inherit (pin) url sha256;
+  };
+in
+
+import nixpkgsSrc {
+  inherit system;
+  config = {
+    allowUnfree = true;
+  };
+  overlays = []; # prevent impure overlays
+}

shell.nix

@@ -1,12 +1,12 @@
-with (import <nixpkgs> {});
+{ pkgs ? (import ./nix/pinned-nixpkgs.nix {}) }:
 
+with pkgs;
 stdenv.mkDerivation {
   name = "env";
   buildInputs = [
     gcc-arm-embedded
-
     (
-      python38.withPackages(ps: [
+      python3.withPackages(ps: [
         ps.intelhex
         (ps.callPackage ./nix/adafruit-nrfutil.nix {})
       ])