CI: Change docker image version to 18.04
- The build servers run ubuntu 18.04 with python 3.6.x, while the CI builds use a 20.04 image with python 3.8.x .This leads to problems with the version of the pickle format used resulting in sstate-cache artifacts generated by the CI build being unusable by devs running bitbake on the build servers. To have compliant versions the CI docker container gets set to 18.04