npm warns about `--global` deprecation

When this plugin is installing packages from `.default-npm-packages`, it uses the `-g` flag. However, in new versions of npm, this gives a deprecation warning:
```
npm WARN config global `--global`, `--local` are deprecated. Use `--location=global` instead.
```
Looking at the source of this plugin, this should be an easy fix - I'm more worried about whether swapping flags here would be a breaking change for older npm versions?