[ci] Install cmake and ninja from apt ci Dockerfile

[mcbarton]

In the ci Dockerfile it says cmake and ninja are fetched from Github releases since Ubuntu 18.04 doesn't have a new enough version to build llvm. The Dockerfile now uses Ubuntu 20.04, so this might no longer be needed, and these can be got from apt, simplifying the Dockerfile.

[mcbarton]

Not sure why my PR caused apt to now require the geographic location. Will convert to draft until I work out why