Add desktop binary exports to the CI pipeline

Export win-x86, win-x64, linux-x64 and mac-universal builds headless
from the godot-builder image and upload them as ReleaseAssets
(<project>-<version>-<target>.zip, single root folder). Mac is one
universal (intel + arm) build with Godot's built-in ad-hoc codesign;
per-arch export is not possible with the official export templates.
ETC2 ASTC texture import is enabled because the arm64 slice requires it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

project.godot

@@ -47,3 +47,8 @@ confirm={
 "events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":32,"physical_keycode":0,"key_label":0,"unicode":32,"echo":false,"script":null)
 ]
 }
+
+[rendering]
+
+; needed for the universal (arm64) macOS export in CI
+textures/vram_compression/import_etc2_astc=true