Add example of downloading rerun c++ lib via pixi

[jleibs]

• Taking over from https://github.com/rerun-io/cpp-example-opencv-eigen/pull/28

The goal is to also show how to build an example using librerun-sdk from the conda environment.

I ended up simplifying the script quite a bit through the indroduction of environment variables.

[jleibs]

@Wumpf would be helpful if you could verify this works on windows