CI fails sometimes with "Multiple Unity instances cannot open the same project."

[falko17]

Since we moved the CI to sts5 in #813, the builds/tests sometimes fail with the following error:

Examples of failing runs:

• https://github.com/uni-bremen-agst/SEE/actions/runs/12865640192/job/35866680586?pr=820
• https://github.com/uni-bremen-agst/SEE/actions/runs/12865640192/job/35866680967?pr=820
• https://github.com/uni-bremen-agst/SEE/actions/runs/12865640192/job/35871238156?pr=820
• And a successful run: https://github.com/uni-bremen-agst/SEE/actions/runs/12865640192?pr=820

It sounds like the concurrent runners somehow interfere with each other, but this shouldn't happen for multiple reasons:

• The four runners are all in completely separate directories, with their own copy of the project.
• The Unity steps are isolated using Docker containers, that should (in theory) have no idea about what other Docker containers are doing. Clearly, this assumption is wrong somehow.

The cause is probably related to the way Unity detects whether a project is open multiple times.
Perhaps it's a problem that we're re-using the Library folder, or Unity just detects this based on the project name/ID instead of the path. A deeper investigation into the cause is necessary here.

[falko17]

Should hopefully be fixed by the new commit for https://github.com/uni-bremen-agst/clone-see-action, but I'll keep this open for a while until we're sure the error doesn't re-occur.