Workflows: Use SHA-1 for third-party actions

[julien-deramond]

This PR suggests to use SHA-1 references instead of versions for third-party actions we rely on.

Source: https://docs.github.com/en/actions/how-tos/security-for-github-actions/security-guides/security-hardening-for-github-actions

Pin actions to a full length commit SHA

Pinning an action to a full length commit SHA is currently the only way to use an action as an immutable release. Pinning to a particular SHA helps mitigate the risk of a bad actor adding a backdoor to the action's repository, as they would need to generate a SHA-1 collision for a valid Git object payload. When selecting a SHA, you should verify it is from the action's repository and not a repository fork.

Please double-check that the sha-1 I've used are really the latest/correct versions in the upstream repositories. It's a bit long to do, but we'll do it only one time.

Indeed, Dependabot is apparently able to handle SHA-1 as well as you can see at https://github.com/twbs/bootstrap/pull/41594/files. I've added the corresponding versions in comment as I saw that Dependabot was able to update this comment as well :)