update quick-install.sh

[slycordinator]

• Made compatible with any posix shell
• Moved command running into a run_cmd function
  • run_cmd will check the userid and if it's not 0/root, then it will run the command with sudo
  • This fixes the issue that the script previously would fail for a root user who doesn't have sudo installed and that it's simply unnecessary to run sudo for root user
• Moved all commands out of the apt_fast_installation funtion as the function isn't really necessary.

Also, this removes the "sudo bash -c", which unnecessarily starts up another shell

[Lasall]

Thank you so much for your PR. I merged it with c2cd0a0