The main benefit of pinning GitHub actions is the determinism it brings
in terms of what version of a given action will be executed. This is
a step towards having hermetic builds.

Once pinned to a commit, dependabot will automatically issue PRs to update
to newer versions.

Pinned versions is the only security metric from OpenSSF scorecard that
this repository currently have a zero score.

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>