distro/rhel90: ensure grub2.Config.Default is preserved

The `grub2.Config.Default` key is owned by `NewGrub2StageOptionsUnified`
and thus needs to be preserved in case there is a `Grub2Config` setting
present in the image configuration (`ImageConfig`).

internal/distro/rhel90/pipelines.go

@@ -556,7 +556,11 @@ func osPipeline(t *imageType,
 
 		if cfg := imageConfig.Grub2Config; cfg != nil {
 			if grub2, ok := bootloader.Options.(*osbuild.GRUB2StageOptions); ok {
-				grub2.Config = cfg
+				// grub2.Config.Default is owned and set by `NewGrub2StageOptionsUnified`
+				// and thus we need to preserve it
+				if grub2.Config != nil {
+					cfg.Default = grub2.Config.Default
+				}
 			}
 		}