The mariadb-plugin-aws-key-management-10.2 deb package for Ubuntu bionic has the following dependencies:
libcurl3 and libcurl4 conflict. They cannot be both installed at the same time. So the package refuses to install. And you don't want to install libcurl3 on Ubuntu Bionic because then you won't be able to install curl.
The default libcurl on Ubuntu Bionic is libcurl4 so that is what is installed on the Ubuntu bionic builders in buildbot. But for some reason the aws key management plugin still insists on having a dependency on libcurl3.