nodejs: download prebuilt add-on at runtime

packages/simplex-chat-nodejs/jest.config.js

@@ -1,3 +1,8 @@
+const path = require("path")
+
+// Tests load the locally built add-on instead of downloading the released one.
+process.env.SIMPLEX_ADDON_PATH ??= path.join(__dirname, "build", "Release", "simplex.node")
+
 module.exports = {
   preset: "ts-jest",
   maxWorkers: 1,