Upload internal packages to dev subdirectory for pre-releases

[brynpickering]

It's useful to make intermediate dev releases available to users (e.g. for stress testing), but they should be appropriately flagged both on github and on packages.arup.com.

On github, you can make pre-releases which works well for dev releases. We could possibly trigger the upload action conditional to the type of release, with pre-releases uploading to a different subdir in the packages directory (e.g. packages.arup.com/conda-dev).