Allow executable to contain spaces

NVM has a special script which loads the correct node version before executing a command.
The syntax for this is `/usr/local/nvm/nvm-exec ...`, so `nvm-exec npm list --json` for example.
But previously when specifying `executable='/usr/local/nvm/nvm-exec nvm'` this would not work because the string was treated as one executable.
Merge pull request #7798 from koenpunt/npm-nvm-exec

Allow npm executable to contain spaces