Set the right environment info for headless builds

Summary: The isHeadlessBuild flag was not properly set.

Reviewed By: antonk52

Differential Revision: D45728435

fbshipit-source-id: 3616c4358114d4f3d96372766dabf48b27b44333

desktop/flipper-server/src/index.tsx

@@ -117,6 +117,7 @@ async function start() {
     argv.launcherSettings,
     keytar,
     'external',
+    true,
   );
 
   exitHook(async () => {