Using Python_jll with PyCall

Is there a reason there's not an easy way to use Python_jll's Python with PyCall, something like `ENV["PYTHON"]="Python_jll"` or something?

As it stands to get the linking right you can do

```julia
LD_LIBRARY_PATH=$(julia -e "using Python_jll; print(Python_jll.LIBPATH[])") \
PYTHON=$(julia -e "using Python_jll; print(Python_jll.python_path)") \
julia -e 'using Pkg; Pkg.build("PyCall")'
```

and then you need `using Python_jll, PyCall` but this could presumably be automated here.

Would a PR adding this be something of interest?