Fix install for Julia <=1.6

[MilesCranmer]

If the Julia project is a shared env, the variable JULIA_PROJECT will only be set to @pysr-{version} if Julia >= 1.7. Otherwise, it will not be set, and the activation performed explicitly.

This also includes a cleaner version check which doesn't require Julia to be started.

@mkitti do you think you could take a look at this?

[mkitti]

The work around here is to give the an absolute path as the JULIA_PROJECT

[MilesCranmer]

Any idea how to get that information in an OS-independent way, without loading Julia? I took a look in JuliaInfo.load() but there doesn't seem to be any relevant paths.

[mkitti]

We could use subprocess.

>>> import subprocess
>>> julia_home = subprocess.run(["julia", "-e print(homedir())"], capture_output=True).stdout.decode()
>>> julia_home
'C:\\Users\\kittisopikulm'
>>> julia_env_dir = subprocess.run(["julia", "-e using Pkg; print(Pkg.envdir())"], capture_output=True).stdout.decode()
>>> julia_env_dir
'C:\\Users\\kittisopikulm\\.julia\\environments'

[MilesCranmer]

Good idea, thanks

[MilesCranmer]

Cool, it's working! Thanks @mkitti!