PR's from external users can't fetch the Private QuantEcon themes

[mmcky]

When a PR is raised by an external user (i.e. not QuantEcon python-lectures access team list) the private theme is not able to be fetched in Github Actions.

• figure out how to specify a user for fetching the theme files for previews.

QuantEcon/lecture-python-advanced#120

[mmcky]

actions/checkout#221

[mmcky]

actions/checkout#13

[mmcky]

This can't be done as a fork will never have credentials for netlify deploys and previews.

I have added artifacts in the meantime when a fork is build so the html files could be downloaded and checked. QuantEcon/lecture-python#218