ubuntu_setup.sh installs the wrong yarn

[OlafSzmidt]

Describe the bug
Running ubuntu_setup.sh on my machine installs everything fine (so far it seems) apart from yarn. Running the yarn command afterwards in the game_frontend directory returns this error:

00h00m00s 0/0: : ERROR: There are no scenarios; must have at least one.

This is confusing. I've realised that the wrong yarn gets installed by the bash script. This one:

http://manpages.ubuntu.com/manpages/xenial/man1/yarn.1.html

Instead, we want to add these instead:

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list

To Reproduce
Steps to reproduce the behavior:

1. Run the script on similar environment setups as below.
2. Run yarn in game_frontend.

Expected behavior
Yarn to download all deps from package.json.

Screenshots

Desktop (please complete the following information):

• OS: Ubuntu 18.04
• Browser: N/A
• Terminal: Hyper

Additional context
https://yarnpkg.com/en/docs/install#debian-stable

A NOTE has been added on official yarn docs from Ubuntu 16+ I believe

When running apt-get install yarn on my system I get, as part of the traceback:

Note, selecting 'cmdtest' instead of 'yarn'
The following NEW packages will be installed
cmdtest