Look for `node` before `nodejs`

[wvengen]

When using NVM having a system node package installed on Debian or Ubuntu, execjs always uses the system node. This is because it looks for the nodejs binary first, which is provided by the system binary and not NVM (since it is something introduced by the distribution).

Suggestion: first look for node, then nodejs. Or don't look for nodejs at all (unless an env variable is set).

First brought up in NVM: https://github.com/creationix/nvm/issues/308#issuecomment-166743174