Pin a known good Nixpkgs revision

This revision is used for unqualified `nix-build` uses.

For the previous behaviour, use:

```
nix-build --arg 'pkgs' 'import <nixpkgs> {}' -A kernel_latest
```

Build was tested through cross-compilation on an x86_64 machine.

default.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? (import ./pkgs.nix) {} }:
 
 let pkgs' = pkgs; in
 let

pkgs.nix

@@ -0,0 +1,9 @@
+let
+  sha256 = "0j8pdr9ymk7a2p8pamcbq2rbhlcg923i6abdmdm6496973s5gb34";
+  rev = "e544ee88fa4590df75e221e645a03fe157a99e5b";
+in
+builtins.trace "(Using pinned Nixpkgs at ${rev})"
+import (fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+  inherit sha256;
+})