Supply chain hardening for GitHub Actions

[MadsRC]

Fixes Or Enhances

This PR pins the versions of the various GitHub Actions used throughout this project.

Why pin?

Version pinning is an essential security control to defend against supply chain attacks. Recently, a well-known GitHub Action (tj-actions/changed-files) was compromised to leak secrets. Such an attack would be not be successfull if downstream users had pinned their dependencies. More info can be defined here: https://www.stepsecurity.io/blog/harden-runner-detection-tj-actions-changed-files-action-is-compromised

Additional information about pinning GitHub Actions can be found here: https://www.stepsecurity.io/blog/pinning-github-actions-for-enhanced-security-a-complete-guide

Updates

Dependabot is compatible with pinning GitHub Actions and will submit back PR's with new versions pinned. Additionally it should add a comment to the end of the hash (like I've done in the code) so show exactly what version is being used.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 74.245% (+0.004%) from 74.241%
when pulling dd7011a on MadsRC:supplyChainHardening
into 8592022 on go-playground:master.