Option to update all shared environments after `juliaup update`

[nathanrboyer]

juliaup makes it easy to update Julia. The most annoying thing about updating now is that all your environments need to precompile again. It would be nice if juliaup would do this for me immediately after updating Julia, rather than having to wait for precompilation the next time I try to use one of my environments.

I imagine an automation that runs Pkg.update or Pkg.precomplile on all the environments in ~/.julia/environments/ would be fairly simple. I'm not sure if something more sophisticated could be used to point juliaup to other local environments the user wants to keep up to date.

(This would become even more useful with an automated juliaup update as requested in #57, so everything updates overnight without my input and is ready to use in the morning.)

[davidanthoff]

So, I think we should never run something like Pkg.update on an env, that will change the project, and I think that should never be done as a by-product of anything. To me that always needs to be a user-initiated step per project.

Maybe a precompile step could work, but also not sure.

I think what we want to do around this scenario might change drastically once #1095 and #1111 are around. Given that, I think for now we maybe should do nothing :)