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/xanthous/default.nix

@@ -2,6 +2,9 @@
 , lib ? pkgs.lib
 , ...
 }:
-pkgs.haskell.lib.failOnAllWarnings (
+(pkgs.haskell.lib.failOnAllWarnings (
   pkgs.haskellPackages.callPackage (import ./pkg.nix { inherit pkgs; }) {}
-)
+)) // {
+  # TODO(grfn): Get this passing (see https://buildkite.com/tvl/depot/builds/3055)
+  meta.ci = false;
+}