Fix XLSX bulk library import, broken by #3765

[jnm]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Corrects the name 'writable' is not defined error when attempting to upload bulk library items with an XLSX (Excel 2007+) file

Notes

Internal discussion: https://chat.kobotoolbox.org/#narrow/stream/7-Kobo-UX.2FUI.2FProduct/topic/Library.20bulk.20imports/near/315222

[notion-workspace]

[IFRC] Cannot upload bulk library items with XLSX (Excel 2007+) file