CI: Flaky test got `ERROR: Cannot open resource pack '/tmp/test_project.pck'.`

### Tested versions

N/A

### System information

N/A

### Issue description

The CI ran for my change https://github.com/godotengine/godot/pull/117739, CI results shown here https://github.com/godotengine/godot/actions/runs/23491468077/job/68360664166

and it failed. I went through the process of binary searching to find the issue:

and eventually tested the original version, and it worked fine

### Steps to reproduce

Unsure here, is it possible there was a race condition? From what I could tell, the failing test was unrelated to my changes.

### Minimal reproduction project (MRP)

N/A

Start by inspecting the linked CI job and its failing test logs, then compare the result with the changes in pull request 117739. Determine why `/tmp/test_project.pck` cannot be opened and whether the failure is reproducible; done means identifying a cause and making the CI test reliable.

Written by the indexing model from the issue text.