Fix nightly build failure + Address issue with the self-hosted runner

[hamzaremmal]

Closes #19712

This PR introduce a workaround for actions/runner#2033. This workaround was already introduced in #19703 in some of the jobs.

actions/checkout has an issue filled about this too: actions/checkout#1169

[hamzaremmal]

The true at the end made the CI pass when it should have failed (See https://github.com/lampepfl/dotty/actions/runs/7924243033/job/21635455423?pr=19703). Should we remove it ?

[smarter]

Sure.

[hamzaremmal]

I'll do it in another PR, see if I can remove the reset too.

[hamzaremmal]

I've added git config --global --add safe.directory /__w/dotty/dotty before each call to a git command as a workaround for the mentioned issue above.

[hamzaremmal]

Also, I believe that the issue didn't manifest earlier (and worse) since the workflow "reset" the repository, then runs the actions/checkout action; which then delete the content of the previous reset. Why are we reseting in that case ? Can we just do the checkout step without reseting the repository ?

[smarter]

I think this was to workaround issues with the repository getting stuck in a bad state with submodules, but it might not be necessary anymore as actions/checkout should now be more robust.

[hamzaremmal]

I'll wait for the CI to finish the run and check if we don't have any other silent failure before merging it