We just rolled out a change to set the user agent explicitly:
options("HTTPUserAgent" = paste0("rstudio.cloud R (", paste(getRversion(), R.version$platform, R.version$arch, R.version$os), ")"))
This should be widely available in the next 30 minutes or so.
It will take a bit of time to fill the package cache, so there may still be the occasional package build failure due to running out of memory for packages.
Thanks for bearing with us.