Add dynamically linked binary environment variables to tox config.

[Johannes Maron]

I am always running into issues when I install a fresh py3 environment via tox on Darwin arm64 with homebrew:

tox -re py3

I'd like to extend tox's passenv configuration with

• LIBMEMCACHED,
• GDAL_LIBRARY_PATH,
• and GEOS_LIBRARY_PATH.

All of those are commonly used to locate linked binaries.

I am currently doing those changes locally and always have to remember not to commit them.

Therefore, I'd like to commit them once and merge them into main :P