[CLI] call nvm use before each npm call on CI

[jhnstn]

While testing the prepare command in a Github workflow I noticed that any call to exec.Command seemed to run in a separate process.
The result is that nvm needs to be ran before each npm command. Without that the workflow falls back to the default system node.

Testing

The real test is to see if the changes work on a github workflow but we can test locally to make sure it's working.

Steps to check locally:

• Create a build to some empty directory e.g. go build ~/some-test-directory/gbm-cli
• cd into that directory and create a new empty directory gb
• From the directory created in the last step run CI=true ../gbm-cli release prepare gb {version} --no-tag
• Confirm that command succeeds

[derekblank]

Good catch! LGTM. 🚀