CLI: Fix nvm setup

[jhnstn]

Fixes #240

This sets up the npm command to correctly use nvm.
It also provides a clear space to introduce other node managers.

Testing

Follow the Testing guide to set up the tool to use forked repos.

Prerequisite: nvm installed

Testing with nvm

• Verify nvm is not loaded in bash_profile , e.g.

$ bash -l 
$ nvm  
bash: nvm: command not found

• Make sure your default node in nvm is not v20
• Try running go run main.go release prepare gb {Version}
• Verify that the command reaches the "create PR" confirmation step

Testing without nvm

• Make sure your default node in nvm is not v20
• Try NVM_DIR= go run main.go release prepare gb {Version}
• This should fail with unsupported engine error
• Set the default node to v20 e.g. nvm alias default v20
• Try NVM_DIR= go run main.go release prepare gb {Version} again
• Verify that the command reaches the "create PR" confirmation step

[dcalhoun]

I tested this while using asdf-vm without nvm installed. The script works as expected, utilizing the "system" Node.js.

Whenever my default Node.js is set to the desired version, the script uses it. I may explore how I might tweak my system bash configuration to expose my local, project-specific Node.js version controlled by asdf-vm to the script environment.

I'll let others provide feedback on the nvm support, but this looks ready to merge from. my perspective.

[dcalhoun]

Optimistically approving nvm support even though I did not test it directly. Others can test and provide feedback in retrospect, if desired.