Check whether bazelisk installed

If `bazelisk-version` is not given, this action does not download bazelisk without notice.

So check whether bazelisk installed after setting up bazelisk(bazel).

Start by tracing the GitHub Action's Bazelisk setup path, especially the case where `bazelisk-version` is not provided. Verify how setup currently handles an installed Bazelisk and whether it downloads one silently; done means the action checks for Bazelisk after setup and makes that behavior explicit.

Written by the indexing model from the issue text.