CI: avoid caching gems

Because we don't commit the lockfile, changes aren't picked up and
incorrect caches restored.

This build ran with Rack 3.1.7 and Sinatra 1.0
https://github.com/sinatra/mustermann/actions/runs/10315713911/job/28556440035#step:3:46

That doesn't happen for me in Docker with Ruby 2.6 and:

    sinatra='~> 3.0.0' bundle install

.github/workflows/test.yml

@@ -34,7 +34,6 @@ jobs:
       id: bundle
       with:
         ruby-version: ${{ matrix.ruby }}
-        bundler-cache: true
     - name: "setup-ruby and bundle install outcome: ${{ steps.bundle.outcome }}"
       run: echo ""
     - name: Run tests