Fix Plugin Scripts load twice on startup

[Hilderin]

• Fixes Godot 4.4 throws errors when using await on EditorPlugin #100750

Thanks to @HolonProduction for helping fix this issue.!!

The issue was caused by the plugin script being loaded twice on startup when the same script is used in the plugin's ready method and as an autoload. Autoloads are created before plugins, triggering the first load. When the plugin is enabled, the script is loaded again with CACHE_MODE_IGNORE, causing it to reload while await calls are still running.

I changed the cache mode to CACHE_MODE_REUSE when enabling plugins on editor startup (while the first scan is running).

I tested this with the MRP from #100750, and as suggested by @HolonProduction, I also retested the MRP from #70667. Godot's startup process has changed quite a bit since this issue was first reported. Since #92667, scripts that reference resources can now load even before the first scan, allowing the project to load without any errors or warnings on the first startup without the .godot folder.

I have kept CACHE_MODE_IGNORE when the editor is not in startup mode to prevent potential side effects.

[HolonProduction]

I think you meant to link to #100750, not my PR 😅

[Hilderin]

Totally right! I changed it. Thanks!

[HolonProduction]

Tested with #100750 and couldn't reproduce any errors in ~20 attempts. I was experiencing the issue around every three tries before, so quite confident that this is fixed.

Wasn't able to reproduce #70667 with this as well.

Also think it makes sense to still reload after the initial scan, since that's usually triggered by explicit user actions.

[Repiteo]

Thanks!