improve optimize images workflow

[martvanrijthoven]

failed job: https://github.com/DIAGNijmegen/website-content/actions/runs/2865886921
previous succeeded job: https://github.com/DIAGNijmegen/website-content/actions/runs/2780797879

The failed job did not have a cache:

Run actions/cache@v2
Cache not found for input keys: Linux-node-64052880d0bdd2703b442d6a8f45cd00cd411a122739d39f746bdb7c2a9f7de5, Linux-node-

While in the previous succeeded job, it used a cache. I think somewhere, something has been updated which failed the job.
netlifly-client is not installed with a specific version, though installing an older version did not work: https://github.com/DIAGNijmegen/website-content/actions/runs/2866123267. However, it might be that even an older version was used in the cache.

I have now upgraded netlify-client, node, and sharp library which solved the problem.

Furthermore, this workflow crashed to concurrent remote changes. A manual rerun solves this issue for now.

TODO

• add status badge for optimize images
• backtrack netlify-client version and check if we want to set a specific version
• check how we can prevent workflow crashes due to concurrent changes (check build deploy workflow)