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