Libptr by env

[cjdoris]

Pass the libptr in juliacall from Python to Julia with an env var instead of by setting a secret variable in Main.

We do this because we can set the env var earlier, in particular, so that PythonCall sees it even if it is loaded during jl_init(), which happens when PythonCall is baked into a system image.

I think this resolves the issue in #773.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	573d7b9...	main / 573d7b9...
basic/@py/pydict/init	0.252 ± 0.051 ms	0.249 ± 0.048 ms	1.02 ± 0.28
basic/@py/pydict/pydel	0.254 ± 0.022 ms	0.257 ± 0.02 ms	0.99 ± 0.11
basic/julia/pydict/init	0.227 ± 0.071 ms	0.221 ± 0.071 ms	1.03 ± 0.46
basic/julia/pydict/pydel	0.237 ± 0.046 ms	0.231 ± 0.038 ms	1.03 ± 0.26
gc/full	0.597 ± 0.008 s	0.602 ± 0.007 s	0.991 ± 0.018
time_to_load	2.03 ± 0.0081 s	2.04 ± 0.01 s	0.994 ± 0.0064

Memory benchmarks

	main	573d7b9...	main / 573d7b9...
basic/@py/pydict/init	5.01 k allocs: 0.0764 MB	5.01 k allocs: 0.0764 MB	1
basic/@py/pydict/pydel	5.01 k allocs: 0.0764 MB	5.01 k allocs: 0.0764 MB	1
basic/julia/pydict/init	4.01 k allocs: 0.0612 MB	4.01 k allocs: 0.0612 MB	1
basic/julia/pydict/pydel	4.01 k allocs: 0.0612 MB	4.01 k allocs: 0.0612 MB	1
gc/full	0 allocs: 0 B	0 allocs: 0 B
time_to_load	0.145 k allocs: 0.26 MB	0.143 k allocs: 10.6 kB	25.1

[cjdoris]

@ncudlenco this PR changes how libptr is passed into julia from python, so it should now be picked up if PythonCall is inited during jl_init, which is the case when it is baked into a sysimage. Could you verify if this change is sufficient to use such a sysimage? I'm hopeful that this is an alternative to #773 that doesn't require any sysimage-specific handling.