Memory leaks in CI for 4.0.3-stable

[Bromeon]

One CI job linux-memcheck-4.0.3 is currently failing with memory leaks, and has been commented out (note that the job was wrongly named).

This one runs the current gdext version (compiled against 4.0.3 API) with a Godot nightly (4.1-dev) binary.

I haven't managed to isolate the leaks yet, as they don't seem to follow a 1:1 per-use pattern. For example, calling Array::default() causes a leak, but calling it twice doesn't cause more leaks, despite the stack trace pointing to this allocation. I wonder if this is a systematic problem similar to the dlclose issue from #133.

[Bromeon]

This configuration (run 4.0.3 API under 4.1 runtime) is no longer supported by Godot, see #305 for details.