Pin GitHub Actions to commit

[rxbchen]

This is an automated PR to update actions in this repo. The operation should be no-op, as we are only switching out the version tag with the matching commit SHA.

To align with industry best practices, we are going to pin Github Actions to a specific commit SHA.

To read more about why pinning actions is recommended check here.

To ensure these Actions stay to-do-date, this PR also enables Dependabot automated updates. To read more about this configuration check here.

If allowed, this PR will attempt to auto-merge in ~1 week.