Yarn does not install correctly when 'wrong version' found

In the install.sh file, if yarn is not installed (ie: not part of sources /) the batch file adds it with the command

echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list

But it does not actually try to install it. So the command under it will fail because only adding the package to the sources will
not actually install it.

yarn --version 2>/dev/null && yarn --help | grep checksums >/dev/null # Check it worked (will error if not)

Need to add something like

apt-get install -Y yarn

to the install.sh file to ensure it gets properly upgraded!

Open install.sh and inspect the branch that adds the Yarn APT source when Yarn is missing. Run the installer in a Debian-based environment and verify that Yarn is installed and the existing version/help check succeeds; done means adding the source also results in a working Yarn installation.

Written by the indexing model from the issue text.