chore: pin commits in github actions #452

Some of the Github workflows use actions with semantic version, instead of the commit hash. That has to be changed, so we can use the workflows again. Currently, there is an error interrupting the execution:

Error: The action slackapi/slack-github-action@v2 is not allowed in deepset-ai/haystack-tutorials because all actions must be pinned to a full-length commit SHA.