The CLOUDINARY_ASSETS environment variable is currently used to pass information between the different event handlers of this plugin. Would it make more sense to use a variable at the top-level scope of the main file instead?

One difference is: environment variable values should be strings, whereas the value of this variable is currently an object. Also, environment variables added by plugins can be seen in the build command. In this case, I think the value would be [object Object] since I believe Node.js would serialize it using String().