revert(glittershark): "Temporarily skip grfn's subtree"

This reverts commit 475d41f698. I'd like
my derivations back, please.

Changes necessary to get this working:

- Don't depend on `nixpkgs` being in the NIX_PATH for my website - it's
  not necessary anyway since emacs 27 is mainline now
- .skip-subtrees on things that shouldn't be evaluated anyway
- Get rid of system/pkgs, and move the one thing in there that *wasn't*
  already in third_party (alsi) to third_party
- Drop notifymuch for now - it's not working, and I'll probably get it
  landed in nixpkgs before I manage to get it working
- Add __readTree = true to my systems so they get built.
- explicitly disable ci for xanthous, which is failing to build and had
  been omitted previously

Change-Id: I20f5e81d6eb7ffe040091a08d75d0cb15304f707
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1864
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/glittershark/system/system/default.nix

@@ -1,8 +1,12 @@
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
 rec {
   chupacabra = import ./machines/chupacabra.nix;
 
+  chupacabraSystem = (pkgs.nixos {
+    configuration = chupacabra;
+  }).system // { __readTree = true; };
+
   rebuilder =
     let
       depotPath = "/home/grfn/code/depot";