Raise an error if providing environment variables to shinyapps.io

[mmarchetti]

Environment variables are silently ignored when deploying to shinyapps.io.

Does shinyapps.io just silently ignore the --environment parameters? Can this error informatively so that you don't have to read the docs to figure out why the app isn't working as expected?

Originally posted by @nealrichardson in https://github.com/rstudio/rsconnect-python/issues/438#issuecomment-1614943782