Add a package to an already installed tools env

[nixjdm]

Summary

Follow-up from #11435

I don't see a way to add a tool to an already existing env. That would be very handy, particularly in the case of adding libraries over time to an ipython-based env. Otherwise a potentially long, growing chain of --with parameters would be needed.

I personally reach for ipython and a few other things frequently, outside of projects, for many things. I'd love to grow that particular tool env to have many of the libs installed I regularly reach for, but not have it be a defined project.

Example

No response

[zanieb]

This is intentional. The tool environments are intended to be immutable. I'm very hesitant to add this.

[zanieb]

There's commentary on this in the original RFC, e.g., #3560 (comment)