library/library_checkout/views/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data>
        <template id="assets_backend_checkout" inherit_id="web.assets_backend" name="assets_backend_checkout">
            <xpath expr="script[last()]" position="after">
                <script type="text/javascript" src="/library_checkout/static/src/js/demo.js"/>
            </xpath>
        </template>
    </data>
</odoo>