How to call R as a non-RStudio session from RStudio?

I expected callr::r call R as a non-RStudio session, but it doesn't.
Is there any way?

callr::r(function() Sys.getenv("RSTUDIO"))
# returns "1" if run on RStudio

I am interested in this topic because of an issue, "parallel::makeCluster() freezes RStudio on macOS with R 4.x," and its workaround:

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