Hash-pin workflow GitHub Actions

[Pedro Kaj Kjellerup Nacht]

Hey, it's Pedro (see #779 and #781) and I'm back with a new security suggestion!

When developing with CI workflows, it's common to version-pin dependencies (i.e. actions/checkout@v3). However, version tags are mutable, so a malicious attacker could overwrite a version tag to point to a malicious or vulnerable commit instead.

Pinning workflow dependencies by hash ensures the dependency is immutable and its behavior is guaranteed.

These hashes can be automatically updated by dependabot. Whenever a new version of an Action is released, you'll receive a PR updating both its hash and the version comment (see this repo as an example).

I'll send a PR pinning the Actions along with this issue.