nix: use package from module in default

This makes overriding hakurei easier. Also avoids building hakurei twice since nix does that for some reason.

Signed-off-by: Ophestra <cat@gensokyo.uk>

options.nix

@@ -1,8 +1,15 @@
 packages:
-{ lib, pkgs, ... }:
+{
+  lib,
+  pkgs,
+  config,
+  ...
+}:
 
 let
   inherit (lib) types mkOption mkEnableOption;
+
+  cfg = config.environment.hakurei;
 in
 
 {
@@ -43,7 +50,10 @@ in
       sharefs = {
         package = mkOption {
           type = types.package;
-          default = packages.${pkgs.stdenv.hostPlatform.system}.sharefs;
+          default = pkgs.linkFarm "sharefs" {
+            "bin/sharefs" = "${cfg.package}/libexec/sharefs";
+            "bin/mount.fuse.sharefs" = "${cfg.package}/libexec/sharefs";
+          };
           description = "The sharefs package to use.";
         };