Automate GHG snapshot

Due to the nature of the [current GHG calculation](https://github.com/SchmidtDSE/plastics-prototype/blob/main/js/exporters.js#L136), the [GHG snapshot](https://github.com/SchmidtDSE/plastics-prototype/blob/main/image_gen/plastics_sankey/data/ghg_snapshot.csv) has to be updated manually. Though a minor inconvenience to future developers, this may be resolved in a future iteration of the software.