fix(wpcarro/emacs): Support OSX build of Emacs

**TL;DR:**
Most of these changes predicate behavior on the platform:
- At buildtime this is `localSystem == $something`. (`localSystem` is from
  `readTree`)
- At runtime this is `(memq window-system '(ns))`.
- Add `coreutils` so `dired` listing with `--group-directories-first` works
  because that flag depends on the GNU version of `ls`.

**Background:**
I need to support a bunch of OSX users at $WORK. As such, I'm planning
on using my MBP for the next few weeks to build empathy for our
userbase and polish some currently rough edges.

If I'm going to get an serious work done, I need my Emacs setup. Step
one is making sure it can build and run.

Change-Id: I918efccfa5f149e218aeea476c2c7df1c7b64ae8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7309
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/common.nix

@@ -44,7 +44,7 @@ in
   # machines.
   shell-utils = with pkgs; [
     bat
-    dig
+    coreutils
     direnv
     diskus
     emacs
@@ -55,13 +55,11 @@ in
     gnupg
     htop
     jq
-    mkpasswd
     nmap
     pass
     python3
     rink
     ripgrep
-    sysz
     tldr
     tokei
     tree
@@ -71,5 +69,14 @@ in
     # wpcarro.tools.simple_vim
     xclip
     zip
-  ];
+  ] ++
+  (if pkgs.stdenv.isLinux then [
+    mkpasswd
+    sysz
+    # This depends on compiler-rt-libc-10.0.1, which is marked as broken on
+    # aarch64-darwin, but depot sets `allowBroken = true`, which means any
+    # build that depends on dig will fail on OSX (e.g. emacs).
+    # https://cs.tvl.fyi/github.com/NixOS/nixpkgs@e9b195248c6cd7961a453b10294aea9ab58e01b4/-/blob/pkgs/development/compilers/llvm/10/compiler-rt/default.nix?L122
+    dig
+  ] else [ ]);
 }