Fix broken built-in script reloading

[KoBeWi]

Fixes #92157
I have no idea what this changes, I only figured that doing this will fix both the error and reload issue.

If default reload() is used, this condition

godot/modules/gdscript/gdscript.cpp

Line 741 in 40b4130

ERR_FAIL_COND_V(!p_keep_state && has_instances, ERR_ALREADY_IN_USE);

will prevent properly loading the script for reasons unknown.

[huwpascoe]

I have no idea what this changes

This PR is covering up whatever the real bug is, there should be a comment like // TODO: I have no idea why this fix works if it's gonna be merged!

[akien-mga]

Couldn't get reviews in a timely manner, but since this fixes a regression, I'll yolo merge and we'll see if anything breaks (and be prepared to revert this if it's problematic).

[akien-mga]

Thanks!

[permelin]

A comment for the future in case this turns out to break something.

I've been running this same fix locally in my own branch since April, but for a different reason. It fixes another problem:
If I changed a .gd file outside the editor, while the editor was running, but without an active LSP connection, I also got ERR_ALREADY_IN_USE and I had to restart the editor for it to reload the script.