refactor(tazjin/emacs): Switch to sending mail via lieer

This drops the msmtp requirement from my configuration; there's still
some cleanup to be done but I need to double-check this in a few
environments first.

Change-Id: I298f4ff77b45cb214fbccee84e9bbd861508d11a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3132
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/nixos/frog/default.nix

@@ -1,7 +1,7 @@
 { depot, lib, pkgs, ... }:
 
 config: let
-  inherit (depot.third_party) lieer;
+  inherit (pkgs) lieer;
 
   quasselClient = pkgs.quassel.override {
     client = true;