Fix invalid GitHub Action YAML files

[lslezak]

Problem

• The GitHub Actions fail with this error: The workflow is not valid. .github/workflows/weblate-update-pot.yml: Unexpected tag '!github.event.repository.fork'

Solution

• From the documentation: You must always use the ${{ }} expression syntax or escape with '', "", or () when the expression starts with !, since ! is reserved notation in YAML format.
• See https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/using-conditions-to-control-job-execution