Don't delete running Julia install

From slack:

> I updated to 1.8.1 in a terminal, but had 1.8.0 running in another terminal. Then I did a Pkg operation there and I got a process failed error when it tried to shell out to /Users/eph/.julia/juliaup/julia-1.8.0+0.aarch64/libexec/7z. IMO it would be nice to not immediately delete the old version but rather clean it up later when it is not in use.

One thought as to a solution:

> since I’m launching julia via juliaup, maybe it can be aware of which versions are running, e.g. by writing a PID file or such

Start by reproducing the upgrade scenario with Julia 1.8.0 running while juliaup installs or selects 1.8.1, then performs a Pkg operation that invokes the old version's 7z executable. Investigate how juliaup tracks installed versions and cleanup timing; done means an in-use version is retained and can complete its operation, while unused old versions are eventually cleaned up.

Written by the indexing model from the issue text.