Update actions/cache and Run Tests on PRs

[mkasberg]

Our build and deploy workflow was using an outdated version of actions/cache that's no longer supported. That can be fixed by updating to actions/cache@v4. No interface changes are required in our YAML.

We also need a way to make sure this works before merging the PR (if we don't want to debug a workflow after merge). The simplest way to do this is to extract our jekyll build steps into a github action. We can write a simple test workflow that just does a jekyll build, and we can use the same jekyll-build action in our existing deploy workflow.

[JohnRDOrazio]

This works for me. As long as we get the build up and running again. I would however suggest adding the ruby version to the cache key:
the cache key is currently ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}. A Ruby ABI change (e.g. 3.2 → 3.4) with an unchanged Gemfile.lock would restore gems with native extensions compiled against the old Ruby — likely to crash at load. Expanded the key to hashFiles('**/Gemfile.lock', '.ruby-version') so future Ruby bumps invalidate the cache cleanly.