gettext-flatpak: Fix only the last pot is updated

[ryonakano]

Fixes only the last pot file is updated when there are multiple pot files.

ryo@b760m:~/work/sgpthomas/hourglass$ TRANSLATION_FILES=$(git ls-files | grep \.pot$)
ryo@b760m:~/work/sgpthomas/hourglass$ echo $TRANSLATION_FILES
po/com.github.sgpthomas.hourglass.pot po/extra/extra.pot
ryo@b760m:~/work/sgpthomas/hourglass$ GETTEXT_TARGETS=$(echo $TRANSLATION_FILES | sed 's/\S*\///g' | sed 's/.pot/-pot/g')
ryo@b760m:~/work/sgpthomas/hourglass$ echo "ninja $GETTEXT_TARGETS"
ninja com.github.sgpthomas.hourglass-pot extra-pot
ryo@b760m:~/work/sgpthomas/hourglass$

[ryonakano]

Tested with Hourglass and confirmed it works:

https://github.com/sgpthomas/hourglass/commits/test-gettext-flatpak/

https://github.com/sgpthomas/hourglass/actions/runs/10623651538/job/29450479408