Latest PROJ versions aren't tested

[greglucas]

We don't have a way to test the latest PROJ version currently, we are dependent on the PROJ version shipped with pyproj.

A few options:

• Install PROJ manually in one of the current runners as an option
• Create a new workflow that installs our environment file and uses that for the testing. This has the added benefit of testing that our environment.yml file works too.

[rcomer]

I like option 2 because it also means we only get the new PROJ once conda-forge has successfully tested pyproj against it (and new GEOS once conda-forge tested shapely against it). So potentially less debugging work for us going forward.

Maybe I'm also just biased as a conda user 😄