Update the PO files as well when updating the POT file

[lslezak]

Problem

• Currently the openSUSE Weblate instance is disabled and does not process the repositories
• So the translations can now only be updated manually directly in the agama-weblate repository
• However, the new texts do not appear in the PO files, only in the agama.pot file. Merging the POT to the translation files is also done by Weblate. (It is optional, I enabled a Weblate plugin for that.) But because it is disabled that does not happen anymore.

Solution

• Do the POT merge directly in the GitHub action, do not rely on Weblate doing that later