We are currently working on a fix for an issue where R 3.6.0
install.packages() requests are not making use of the package cache provided for rstudio.cloud. It appears that with R 3.6.0 there was a change to the user-agent used when making these requests (see the commit here).
In the meantime, if you choose to use R 3.6.0, the package cache will unfortunately not be used which will lead to your packages being compiled from source, and will likely result in out-of-memory issues for larger package compilations. We apologize for the inconvenience, and will report back here as soon as a fix is in place!