Add: script for update flake lock

[a-kenji]

Description

Take commands out of the action.yml file, and put it in a dedicated
shell script. I worked with actions recently and figured out that $GITHUB_ACTION_PATH refers to the root of the action runner, and remembered that taking the commands out of the yaml was a goal of this project.

Checklist

• Tested functionality against a test repository (see "How to test changes")
• Added or updated relevant documentation (leave unchecked if not applicable)

[cole-h]

Oh, I had no idea that env var existed! I'll take a closer look at this tomorrow, but thanks for opening this PR!

[a-kenji]

Awesome, no rush! Yeah, I just stumbled across the variable by accident, more or less.

[cole-h]

Thanks, looks good! I pushed a commit re-adding our shellcheck CI and an action that will use ourselves to keep our own flake.lock up to date :)

Thanks a ton for reading about this env var and remembering us ❤️