wbt_install: remove downloaded zip file when exiting function

[cstepper]

Hi,

thanks for this great package and the whiteboxtools to use for geospatial analysis.

We bake the whiteboxtools into some docker images (which we use for CI/CD stuff etc.), installing the whiteboxtools not into the R package itself but to some other location (/usr/local/bin)

We think that after installation (or failure during the wbt_install), the downloaded zip file could be deleted from that location (to keep things tidy).

Happy to discuss and contribute.

Cheers, Christoph

[brownag]

Thank you for your contribution!