Add action to prevent changes to release files

[juliapampus]

We tend to accidentally change the files in the release folder. A GitHub action could prevent this: https://github.com/xalvarez/prevent-file-change-action.

- name: Prevent file change
  uses: xalvarez/prevent-file-change-action@v1
  with:
    githubToken: ${{ secrets.GITHUB_TOKEN }}
    pattern: releases/.*
    allowNewFiles: true

allowNewFiles: true to avoid that the action fails when we add a new release.