[YottaDB/DB/YDB#1028] Don't re-install libsodium if already installed
As part of YottaDB/DB/YDB#1028 (closed), ydbinstall.sh
will handle the install
of all plugins. Therefore, if a plugin is already installed, CMake will
not reinstall it.
Previously, the behavior here was that libsodium was always re-installed no matter the previous installation.