Find chia in env if needed

[altendky]

Fixes #80.

Draft for:

• Self review
• Deal with PyPy, one way or the other
• Make plotman runnable without a findable chia
  • I figure it could still be useful for monitoring
  • Maybe require an override, maybe make lack of chia a warning rather than an error

[altendky]

I did run a k25 the whole way through and got a plot out the other end. I dunno, I do think plotman should be able to work without activating the environment but there are various aspects of this that make me a bit itchy. Let me know which bits do that for you.

And yeah, I may yet drop PyPy depending if Chia works with it. Which it probably doesn't.

ping: @ericaltendorf, @jkbecker, @chelseadole

[chelseadole]

Pretty clever solution to a finnicky situation. As is often the case with these kind of issues, there's likely a lot of room for edge cases, but I'm not sure they're worth the time they'd take to suss out, as they're likely quite specific.

The override of the user's environment makes me a bit itchy, but to be fair it's only for the duration of the single command