Scan the filesystem in the first frame when using headless mode

[clayjohn]

Tentative fix for #84460

I tested with a larger project that uses GDExtension. My testing procedure was as follows:

1. delete extension_list.cfg
2. Run godot --path test/project --headless --editor --quit using master (extension_list.cfg was not generated and I got a warning: WARNING: Scan thread aborted...)
3. Run godot --path test/project --headless --editor --quit-after 100 using master (Same results as before. Since this is a larger project, I imagine the delay would need to be larger as well)
4. godot --path test/project --headless --editor --quit using this PR (the extension_list.cfg file is generated right away)

[YuriSizov]

Thanks! David should confirm with his own testing, but this is exactly what the behavior was prior to the recent changes. So it should be good.

[dsnopek]

Thanks! I tested locally and it seems to be working. Waiting to see if it works with godot-cpp's CI as well - see PR godotengine/godot-cpp#1304