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/home/modules/alsi.nix

@@ -1,8 +1,6 @@
 { config, lib, pkgs, ... }:
-let alsi = pkgs.callPackage ../../pkgs/alsi {};
-in
 {
-  home.packages = [ alsi ];
+  home.packages = [ config.lib.depot.third_party.alsi ];
 
   xdg.configFile."alsi/alsi.logo" = {
     source = ./nixos-logo.txt;