weblate-merge-po.yml - fixed PO file validation

[lslezak]

Problem

• The PO file validation in the GitHub Action fails
• See https://github.com/openSUSE/agama/actions/runs/5988472539/job/16243691592#step:9:5

Solution

• Each PO file needs to be checked individually otherwise msgfmt complains about duplicate messages.
• Note: We cannot use find -exec because it ignores errors in the child processes

Testing

• Tested manually