PyCall build silenty fails to use virtualenv if ENV["PYTHON"] set incorrectly.

Hi there, I think it would be good to have a warning if `ENV["PYTHON"]` does not point to a python binary. I was setting it to the location of the virtual environment, without the `bin/python` appended to the end. This caused `PyCall` to build for the `Conda.jl` setup, without reporting why the virtual environment I wanted to use wasn't being picked up. It took me more time than I'm comfortable admitting to realize the problem 😬 .

This sounds pretty simple to fix. If it's agreeable, I'd be happy to write a small PR to add a warning when `ENV["PYTHON"]` (or `ENV["PYCALL_JL_RUNTIME_PYTHON"]`) do not point to a valid binary.

Start by tracing how PyCall handles ENV["PYTHON"] and ENV["PYCALL_JL_RUNTIME_PYTHON"] during its build configuration. Verify the behavior for values that name a virtual-environment directory rather than a Python binary, and consider the work complete when that case produces a clear warning instead of silently selecting another setup.

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