Executing using active kernels anywhere on the computer is confusing and hard to debug

[choldgraf]

When executive a MySTMD document with --execute, MyST will search for any active kernel and use it rather than spinning up a new kernel with the local environment. This happens more-or-less silently.

It is really confusing when you're trying to run Jupyter Book in an isolated environment. For example, say you want to build a book with nox using an environment that's installed locally. But, you've got a Jupyter kernel running anywhere else on your machine. When the book executes, it won't spin up a kernel for the isolated environment, it'll just re-use the existing kernel and whatever things are installed there.

Note: I think we've discussed this before but I couldn't find an issue, so please close and re-direct if there's a better place.