Use system eigen

Dear Jake,

my distro has eigen 3.4.1 https://gitlab.com/libeigen/eigen/-/releases/3.4.1 while USD-Fileformat-plugins wants 3.4.0 – they are certainly interchangeable.

Please add an option to force the eigen system lib instead of downloading it:

`[ 22%] Performing download step (git clone) for 'eigen-populate'`

Thanks in advance!

Start by tracing the build configuration responsible for the `eigen-populate` download step and how it selects Eigen 3.4.0. Identify the option needed to use an installed system Eigen such as 3.4.1 instead. Done means configuration can avoid the git clone and use the system library successfully.

Written by the indexing model from the issue text.