Can't release LanguageTool 5.2 due to unsecure command execution.

[Erikvl87]

The following output is given by our Release Github Action:

Run echo "::set-env name=DOCKER_TAG::$(echo refs/tags/v5.2 | grep -oP '(?<=refs/tags/v).*')"
Error: Unable to process command '::set-env name=DOCKER_TAG::5.2' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

This blocks the release of the v5.2 docker container.