build(action): update node16 to node20

[cameronraysmith]

Description

node 16 is now being deprecated relative to node 20

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: ... 
For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

magic-nix-cache-action/action.yml

Line 36 in 1402a2d

using: "node16"

Checklist

• Tested changes against a test repository
  • see ci run in fork
• Added or updated relevant documentation (leave unchecked if not applicable)
• (If this PR is for a release) Updated README to point to the new tag (leave unchecked if not applicable)

[kubo6472]

I actually jumped the gun on this and thought that main will be definitely running on v20 one hour after this PR was opened. If the tests run okay...

[ConnorBaker]

@kubo6472 do you have permissions to run the test workflow? If not, who should we loop in to get this PR accepted and merged?

[cole-h]

Thanks, seems to work fine!