refactor(tazjin/nixos): pick correct Emacs from a NixOS option

This gives me the ability to override the Emacs per-machine easily.

Change-Id: Id480889c108833b0a11c377a9b1e946900c5aba1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9166
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

users/tazjin/nixos/modules/desktop.nix

@@ -1,5 +1,5 @@
 # EXWM and other desktop configuration.
-{ depot, lib, pkgs, ... }:
+{ config, depot, lib, pkgs, ... }:
 
 {
   services = {
@@ -29,7 +29,7 @@
 
       windowManager.session = lib.singleton {
         name = "exwm";
-        start = "${depot.users.tazjin.emacs}/bin/tazjins-emacs";
+        start = "${config.tazjin.emacs}/bin/tazjins-emacs";
       };
     };
   };

users/tazjin/nixos/modules/physical.nix

@@ -1,16 +1,27 @@
 # Default configuration settings for physical machines that I use.
-{ lib, pkgs, depot, ... }:
+{ lib, pkgs, config, depot, ... }:
 
 let
   pass-otp = pkgs.pass.withExtensions (e: [ e.pass-otp ]);
 in
 {
+  options = with lib; {
+    tazjin.emacs = mkOption {
+      type = types.package;
+      default = depot.users.tazjin.emacs;
+      description = ''
+        Derivation with my Emacs package, with configuration included.
+      '';
+    };
+  };
+
+  config = {
     # Install all the default software.
     environment.systemPackages =
       # programs from the depot
       (with depot; [
         users.tazjin.screenLock
-      users.tazjin.emacs
+        config.tazjin.emacs
         third_party.agenix.cli
         third_party.josh
       ]) ++
@@ -25,7 +36,7 @@ in
         direnv
         dnsutils
         electrum
-      emacs-unstable # emacsclient
+        config.tazjin.emacs.emacs # emacsclient
         exa
         fd
         file
@@ -88,4 +99,5 @@ in
       mosh.enable = true;
       ssh.startAgent = true;
     };
+  };
 }