Fixed XML error in Firefox. Renamed telemetry env var.

scripts/postbuild.js

@@ -2,7 +2,7 @@ require('dotenv').config();
 const sendTelemetry = require('./telemetry');
 
 async function run() {
-  if (!process.env.TELEMETRY_DISABLE) {
+  if (!process.env.DISABLE_TELEMETRY) {
     await sendTelemetry();
   }
 }