fix: Make built-in extension action exit early when building legacy

Signed-off-by: Mahid Sheikh <mahid@standingpad.org>

bpy_addon_build/built_in_actions/extension.py

@@ -37,6 +37,8 @@
 
 
 def main(ctx: BabContext) -> None:
+    if not ctx.is_extension:
+        return
     manifest_path = Path(ctx.current_path, BLENDER_MANIFEST)
     manifest_data = get_manifest_data(manifest_path)
     verify.verify_manifest(manifest_data, manifest_path)