CI: github: update to cache@v5

CI: github: update to cache@v5

github complains about cache@v4:

Node.js 20 actions are deprecated. The following actions are running on
Node.js 20 and may not work as expected: actions/cache@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/