Respect JULIA_PROJECT environment variable if set

I am using the python package `juliacall` and am surprised to find that it ignores `JULIA_PROJECT` environment variable.
[Julia documentation about JULIA_PROJECT](https://docs.julialang.org/en/v1/manual/environment-variables/#JULIA_PROJECT).

It would be great if this standard variable is respected. Currently juliacall opens a new julia environment `pyjuliapkg`

Start by tracing juliacall's Julia environment initialization and how the pyjuliapkg environment is selected, then compare that behavior with Julia's JULIA_PROJECT documentation. Done means a set JULIA_PROJECT value is respected instead of always opening pyjuliapkg.

Written by the indexing model from the issue text.