[FIX] base ir.actions.report order pdfs when attachment_use option selected

Description of the issue/feature this PR addresses:
Order of merged pdf reports when attachment_use = True

Current behaviour before PR:
When in a report the "Reload from Attachment" option was selected
(attachment_use = True), and multiple pdfs were printed, the singular pdfs
were stored in a dictionary and appended to a list and thus merged in a random order.

Desired behaviour after PR is merged:
By mapping the pdf with their source record, and sorting the list with the table _order of
the source records of the pdfs, we seek to have an ordered output pdf.

opw 1915685

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closes odoo/odoo#30402