interactive maps display results of earlier chunks on shinyapps

[andysouth]

When the learnr tutorials are hosted on shinyapps, running later chunks can lead to display of the results of earlier chunks using tmap & mapview.

e.g. https://andysouth.shinyapps.io/intro-to-spatial-r/#section-e.-first-maps-with-tmap

Issue does not happen when run locally.

Does happen in a smaller reproducible example :
https://andysouth.shinyapps.io/test-tmap-cache/

About to submit issue to learnr.