yarn dev currently calls source-nvm.sh twice at the same time. If the version of Node is not installed already, both calls will trigger an install.
The result is that one of the calls "clobbers" the other one's download, and the other call decides that the download failed and it needs to compile Node from source (which takes a long time).
This diff makes sure we call source-nvm.sh at the start (not concurrently), and wait for it to finish before we call the other commands.