Add `cargo` binary to container libraries to support gganimate



The gganimate package requires the gifski package, which has a rust dependency on cargo. Would it be possible to have this dependency added to containers being used on RStudio Cloud?

------------------ RUST COMPILER NOT FOUND --------------------

Cargo was not found on the PATH. Please install cargo / rustc:

 - yum install cargo         (Fedora/CentOS)
 - apt-get install cargo     (Debian/Ubuntu)
 - brew install rustc        (MacOS)

As the projects run under Ubuntu 16.04.5 LTS, we'd need:

apt-get install cargo