imex: make it possible to cancel backup export

Previously shall_stop_ongoing() was never used
in backup export, so cancelling backup export
did not actually stop the process.

src/imex.rs

@@ -614,6 +614,11 @@ async fn export_backup_inner(
             );
             continue;
         }
+
+        if context.shall_stop_ongoing().await {
+            bail!("Backup export cancelled");
+        }
+
         let mut file = File::open(entry.path()).await?;
         let path_in_archive = PathBuf::from(BLOBS_BACKUP_NAME).join(name);
         builder.append_file(path_in_archive, &mut file).await?;