Make simplecov-json as optional dependency

[hsbt]

In ruby/ruby repository, we can't use simplecov-json for make test-bundled-gems.

https://github.com/ruby/ruby/actions/runs/11569147158/job/32202320346#step:14:163

I added workaround for that. I'm happy to release v0.5.1 with this.

[nevans]

👍🏻

I originally considered only loading simplecov if an ENV var is present. I like this better. 🙂

[nevans]

@hsbt I could also make the release, but I don't think you've made a release since I added Trusted Publishing. So it's probably good for you to do it. All you should need to do is:

1. bump the version number (same as it ever was)
2. push the tag
3. Find the workflow run and approve the deployment (and wait a couple of minutes).
4. The GitHub release notes should be auto-generated, and saved as a draft.

Let me know if you hit any issues. Obviously, you can still just create a release the old way, if anything goes wrong.