build: git ignore local environment overrides (#580)

Git ignore for .env.private to enable local environment
overrides as detailed by frontend-build. See:
https://github.com/edx/frontend-build#override-default-envdevelopment-environment-variables-with-envprivate

.gitignore

@@ -20,3 +20,6 @@ logs
 
 # Local package dependencies
 module.config.js
+
+# Local environment overrides
+.env.private