gettext: Follow rebase of flatpak-platform Docker image

[ryonakano]

Part of elementary/flatpak-platform#236

[ryonakano]

I can confirm gettext-flatpak actions works as before in my forked repository. It commits translation template changes to the repository if I add a new translatable string:

(Ignore the deploy workflows are failing; tha's because I didn't feed API key as not relevant with this PR)

[davidmhewitt]

I wonder if it's better to have the Gettext action ensure Python is installed and has the necessary dependencies itself. Then if we need to do changes like this in the future, we only need to make the change in one place.

For example, we could add a setup-python step to the action to ensure Python is installed:
https://github.com/elementary/actions/blob/b6e5cba9fac00456f1075bfff13005ba7f7ac472/gettext-flatpak/action.yml#L16

And use some kind of Python lock file (requirements.txt, poetry, etc...) in the actions repo to ensure we always install the same version of GitPython. I'm a bit nervous about installing things without a fixed version in CI jobs that run often...

https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#caching-packages