[BUG]unbuffer does not work

## User Story

As a lgsm user, I want the unbuffer command to work correctly so that steamcmd update progress will be displayed smoothly, allowing for a better user experience and 3rd party programs to fetch the installation progress more frequently.

## Basic info

* **Distro:** [Debian 10]
* **Command:** [install]
* **LinuxGSM version:** [v21.4.0]

## Further Information

A few years ago, the unbuffer command was implemented into core_dl.sh (see #275 ). Unfortunately it doesn't seem to work anymore. Steamcmd progress output is only being displayed every couple of minutes (I read something about 4kb in text per flush), often resulting in no progress being displayed at all (at least for smaller servers that are downloaded relatively fast).

## To Reproduce

Steps to reproduce the behaviour:
1. Run install for a server using steamcmd.
2. Watch the steamcmd update progress being displayed unsmoothely or not at all

## Expected behaviour
steamcmd progress should be updated every second or so.