Hardcoded CURL_CA_BUNDLE interferes with custom SSL certificates

[alexvorobiev]

The bin/R script has this line

export CURL_CA_BUNDLE=${R_HOME_DIR}/lib/microsoft-r-cacert.pem

This breaks the --vanilla mode of running R when any kind of connection is needed when custom SSL certificates (common on enterprise networks) are involved. One example is publishing from RStudio Pro server to RStudio Connect. In the vanilla mode the files like Rprofile.site are ignored so we can't unset the variable there.