It looks like that I was trying to execute the wrong yarn, because simply running sudo apt install yarn on my Ubuntu 18.04 gave me yarn from cmdtest.
So I solved by uninstalling it:
sudo apt remove yarn
And by installing it as the official website explains, which in my case (Ubuntu 18.04) it was the following:
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
sudo apt update && sudo apt install yarn
yarn