plugin/node.js: don’t overwrite already-set variables

[gaelicWizard]

Description

plugin/node.js: don’t overwrite environment variables that the user may have already set:

• $NENV_ROOT
• $NODENV_ROOT
• $NVM_DIR

Alsö, no need to check if directory exists before calling pathmunge as it already does so.

Motivation and Context

Just helping out 👍

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.