CI: Update actions/checkout to v6.0.2

[RytoEX]

Description

CI: Update actions/checkout to v6.0.2

Also pin the action to a commit hash.

Motivation and Context

Want to update used Actions and avoid deprecation warnings.

How Has This Been Tested?

It hasn't been tested on this repo, but we've updated actions/checkout elsewhere without issue.

Types of changes

• CI

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.