Support offline / no-index installs

We occasionally see build failures due to problems accessing the npm server. While we could run a local npm server instance ourselves, doing so adds unwanted complexity.

The Python world has a simple solution: save a copy of the used packages to a local directory or web accessible directory listing, then install using:

```
$ pip --no-index --find-links=/path/or/url ...
```

The package manager will then simply fetch the desired packages from the local directory or from the links extracted from the returned html directory listing. See: https://pip.pypa.io/en/stable/reference/pip_wheel/?highlight=find-links#cmdoption-f

This also provides a simple way to publish private packages within your organisation.