fix(.nix): allowUnsupportedSystem config for nixpkgs

On Darwin/MacOS:

```
error: Package ‘libcap-2.48’ in /nix/store/f7h0k1cd902y5v7f5x16pkfm2c02k4lx-source/pkgs/os-specific/linux/libcap/default.nix:59 is not supported on ‘x86_64-darwin’, refusing to evaluate.

a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
   for a single invocation of the nix tools.

     $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1

b) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnsupportedSystem = true; }
in configuration.nix to override this.

c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnsupportedSystem = true; }
to ~/.config/nixpkgs/config.nix.
```

default.nix

@@ -1,5 +1,8 @@
 { nixpkgs ? ./nix/nixpkgs-stable.nix
-, pkgs ? import nixpkgs
+, pkgs ? import nixpkgs {
+    # This is a hack to work around something requiring libcap on MacOS
+    config.allowUnsupportedSystem = true;
+  }
 }:
 let
   src = pkgs.nix-gitignore.gitignoreSource [

nix/nixpkgs-1909.nix

@@ -5,4 +5,4 @@ let
     sha256 = srcDef.sha256;
   };
 in
-import nixpkgs { overlays = [ (import ./overrides.nix) ]; }
+args@{...}: import nixpkgs (args // { overlays = args.overlays or [] ++ [ (import ./overrides.nix) ]; })

nix/nixpkgs-stable.nix

@@ -5,4 +5,4 @@ let
     sha256 = srcDef.sha256;
   };
 in
-import nixpkgs { overlays = [ (import ./overrides.nix) ]; }
+args@{...}: import nixpkgs (args // { overlays = args.overlays or [] ++ [ (import ./overrides.nix) ]; })

shell.nix

@@ -4,7 +4,10 @@
   # on CI). Only when this is enabled, Rust nightly is used.
   isDevelopmentShell ? true
 , nixpkgs ? ./nix/nixpkgs-stable.nix
-, pkgs ? import nixpkgs
+, pkgs ? import nixpkgs {
+    # This is a hack to work around something requiring libcap on MacOS
+    config.allowUnsupportedSystem = true;
+  }
 }:
 
 let