Fix: Flutter doctor が permission denied で失敗 (Linux)

$ flutter doctor
touch: cannot touch '/opt/flutter/bin/cache/.dartignore': Permission denied

$ sudo flutter doctor
   Woah! You appear to be trying to run flutter as root.
   We strongly recommend running the flutter tool without superuser privileges.

$ sudo usermod -a -G flutterusers <your-user>
$ # alternatively:
$ #sudo gpasswd -a <your-user> flutterusers

$ flutter doctor
   Building flutter tool...

$ sudo chown -R <your-user>: /opt/flutter