Allow overriding src

src/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, bash, coreutils, nodejs, perl, stdenv, writeText }:
+{ lib, coreutils, nodejs, stdenv, src ? ./. }:
 
 with lib;
 
@@ -23,9 +23,9 @@ let
 
   # Directory of just the files needed to run Yarn.
   yarn-closure = cleanSourceWith {
-    src = ./.;
+    inherit src;
     filter = let
-      srcStr = toString ./.;
+      srcStr = toString src;
       srcRel = removePrefix "${srcStr}/";
     in path: type:
       elem "${type}:${srcRel path}" yarn-closure-entries;
@@ -71,7 +71,7 @@ let
 
 in stdenv.mkDerivation {
   name = project-name;
-  src = ./.;
+  inherit src;
 
   # Tell node-gyp to use the provided Node.js headers for native code builds.
   npm_config_nodedir = nodejs;