feat(env): allow nodejs path to be set by the environment

[sheeldotme]

Pretty straight forward amendment here to allow node's path to be set via an environment variable. One use case, of many, is when it is desirable to spin up a new work environment with neovim + coc. coc's node path dependency can now be preconfigured without parsing / writing configuration files. This code change allows one to still override the env variable with g:coc_node_path and as a last fallback still use node.