[client-app] actually npm install inside the /apm dir with env vars

Summary:
We weren't actually running npm install inside of apm. This meant that
/apm/node_modules was empty causing the build to fail. This will install
with the proper env required for old apm

Wow we need to get rid of this thing soon…

Test Plan: Run `npm install`

Reviewers: jerm, spang, juan

Reviewed By: juan

Differential Revision: https://phab.nylas.com/D3986

scripts/postinstall.js

@@ -86,7 +86,7 @@ async function electronRebuild() {
     console.log("\n---> No client app to rebuild. Moving on")
     return;
   }
-  await npm('rebuild', {
+  await npm('install', {
     cwd: path.join('packages', 'client-app', 'apm'),
     env: 'apm',
   })