Split manual and scheduled system tests workflows

[MakisH]

The latest components workflow is currently being executed both every night and manually.

Since the manual runs often fail (as they are meant to when debugging), status badges such as the one in the preCICE repository are sometimes reporting a failure that does not correspond to the scheduled run. Moreover, looking back at the history (to see when something started failing) is complicated.

This PR:

• splits the system-tests-latest-components.yml to system-tests-manual.yml and system-tests-nightly.yml,
• renames the system-tests-extra.yml to system-tests-weekly.yml for consistency,
• replaces the triggering of workflows from absolute paths+branch to relative paths, hoping to make debugging of the workflows easier:

- uses: precice/tutorials/.github/workflows/system-tests.yml@develop
+ uses: ./.github/workflows/system-tests.yml

@PranjalManhgaye since I cannot easily test before merging, do you see anything I missed / anything wrong?

Checklist

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.

[PranjalManhgaye]

Overall, it looks good to me.