Fix openlibrary-data-loader.sql query

[danmichaelo]

Fixes #12

• limit 1 in the inner query causes the script to only import the first file from the filenames list (using Postgres 15), since the limit is applied after unnesting the filenames.
• We can fix this by moving unnest(filenames) AS filename to the outer query, so that the limit is applied before unnesting the filenames.

[DaveBathnes]

Thanks for this! Looks great - much appreciated. The repository isn't well maintained so these fixes are really helpful