Increase the frequency of the Private Projects CI test runs

[MilanKlausz]

Due to the DGCode Private Projects being a repository on GitLab, new commits can't trigger the GitHub testing workflow. (Or at least I wasn't able to find a straightforward way to do it.) The current solution is to simply run the testing workflow of the private projects on a weekly basis.

• The Idea for an upgrade is to increase the frequency of the workflow runs, but making sure that it only does the building/testing process if there was a new commit since the beginning of last test workflow run.
• The test runs should clearly indicate the lates commit of DGCode Private Projects repo, to see which version was tested.

Note: if all the workflow strategy matrix elements (env setups) are among the ones used in the validation workfow, we don't really need to enforce running the workflow weekly to keep the caches alive, because the cron job of the validation workflow will cover that purpose.

[MilanKlausz]

Created a new workflow (52c2a94) that runs hourly, and does the following:

• checks the timestamp of the lates commit in the Private Projects through the GitLab API
• checks when was the last time that this workflow checked for new commits (the unix timestamp of the time when the API was called is saved in a file, upoaded as a artifact)
• compares the two dates, and calls the private projects test workflow if there is new commit since the last check.