When you install a new node version using nvm and then used npm to install yarn, you need to reinstall the yarn for the new node version.
Try:
nvm install 8.11.3
nvm use 8.11.3
npm install -g yarn
This will install yarn in:
.nvm/versions/node/v8.11.3/
You can then switch between 8.11.0 and 8.11.3 and your yarn will still work.