src-cli: Clean up partially downloaded ZIP files if user aborts with Ctrl-C

Created by: mrnugget

When you hit Ctrl-C while src-cli is downloading the repository archives for src campaign [preview|apply] the download will be cancelled and the ZIP files will be left on disk in an invalid state. The next run then produces an error that the ZIP file is invalid.

We should handle context cancellation correctly and clean up these files if we haven't downloaded them fully.