Fix: check if nvm exists, upgrade node to v12.20.1 (#743)
Fix: check if nvm exists, upgrade node to v12.20.1 (#743)

The script fails to install nvm because when it already exists, so we need to perform a check to see if nvm is present.
NodeJS upgraded to latest LTS v12.20.1.
Percona-Lab/jenkins-pipelines,Percona-Lab/jenkins-pipelines,Percona-Lab/jenkins-pipelines,Percona-Lab/jenkins-pipelines