Add passthru yarn derivation

This can be used to run any scripts in the package.

src/tmpl/yarn-project.nix.in

@@ -1,7 +1,7 @@
 # This file is generated by running "yarn install" inside your project.
 # Manual changes might be lost - proceed with caution!
 
-{ lib, nodejs, stdenv, fetchurl, writeText, git, cacert }:
+{ lib, nodejs, stdenv, fetchurl, writeText, git, cacert, writeShellApplication }:
 { src, overrideAttrs ? null, ... } @ args:
 
 let
@@ -151,6 +151,13 @@ let
 
     passthru = {
       inherit nodejs;
+      yarn = writeShellApplication {
+        name = "yarn";
+        runtimeInputs = [ nodejs ];
+        text = ''
+          ${yarnPath} --cwd ${project}/libexec/${project.name} "$@"
+        '';
+      };
     };
   });