FR: Introduce `R-CMD-check-offline` workflow?

[IndrajeetPatil]

The R-CMD-check-hard makes sure that all examples and tests that require suggested dependencies use them conditionally and with skip_if_not_installed(....), respectively.

In other words, you can use this workflow to find tests that should have skip_if_not_installed(....).

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Similar to this workflow, I was wondering if it might be a good idea to introduce R-CMD-check-offline workflow, which will make sure that all examples and tests that require internet access are being run conditional on internet access availability.

In other words, you can use this workflow to find tests that should have skip_if_offline().

If you agree, I'd also be happy to make a PR, but I am having a hard time figuring out how to mimic no internet access during R CMD check. Couldn't find anything relevant in {curl}.