Handle environments when upgrading across a minor version bump

This suggestion comes from [here](https://discourse.julialang.org/t/ann-juliaup-preview-julia-version-manager-and-windows-store-installer/61920/2).

I think one option would be that if `juliaup update` detects an update that increases the minor version of Julia, that it copies the older environment file to the new Julia minor version. I think I would limit it to this scenario, i.e. not do anything with downgrades or stuff like that.

One question is whether just copying the env is actually a good user experience? Or would we have to instantiate, build or do other things as well?

Start with the `juliaup update` command and the environment files associated with Julia minor versions. Determine how an update detects a minor-version increase and whether copying the older environment is sufficient or requires instantiation or building. Done means the intended upgrade behavior and user experience are defined, including what happens on downgrades.

Written by the indexing model from the issue text.