Wrong "updated on" date for imported pages

[fsimonis]

Submodules are still shallow-cloned, leading to incorrect "Updated" status of imported pages.

This is a bug in the action itself.

[MakisH]

This issue still holds: all tutorials show the same "updated" date as the last commit to the website. Example: https://precice.org/tutorials-aste-turbine.html

Related issue in the checkout action: actions/checkout#1124

[Aayushman-nvm]

This should be resolved in PR #783. @MakisH could you please review? Happy to make any adjustments if needed.