Update `actions/checkout` in GitHub Actions workflow

@gouttegd is already working on this.

Since May 14, 2026.

I've been seeing these warnings under GitHub Actions workflows that use `actions/checkout@v4` (which is used in ):

> Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/checkout@v4. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Node.js 20 will be removed from the runner on September 16th, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

The fix should be as easy as updating to `actions/checkout@v6`, but it seems like current workflows will stop working without intervention on June 2 (!)