The antithesis image build process uses db-contrib-tool to download ubuntu1804 binaries. Recently platorm support was dropped for ubuntu 1804 on this commit https://github.com/10gen/mongo/commit/1685fecbed14a1307ad3db5ade31d34dfd5b37ef which made db-contrib-tool no longer get these binaries properly. We should just switch to the exact antithesis variant so that we don't risk this variant disappearing in the future.

We should also eventually upgrade to ubuntu 2204. This is tracked in SERVER-80448