unixbench: libperl5.28 does not exist on ubuntu-18.04

[jallen-amd]

7cd78129d06e3c8fd47eaab4ff seems to have introduced an issue with ubuntu-18.04. The patch changes the install dependencies for ubuntu to perl-modules-5.28 and libperl5.28 from 5.26. However, ubuntu-18.04 repos seem to only contain version 5.26. Is ubuntu 18.04 not a supported distro? If it is supported, it seems that we will need a way to specify to install whichever version of the package is available. Ideally, it would be nice to be able to specify something like "perl-modules >= 5.26", but the current method of installing packages doesn't seem to be equipped to handle that sort of thing.