PS-8502: Building issue with ccache on Azure

1. Remove `##vso[task.prependpath]`.
2. Use `-DCMAKE_C_COMPILER_LAUNCHER=ccache` and `-DCMAKE_CXX_COMPILER_LAUNCHER=ccache`.
3. Use `-DCMAKE_C_COMPILER=$SELECTED_CC` and `-DCMAKE_CXX_COMPILER=$SELECTED_CXX`.
4. Introduce `$PARENT_BRANCH` for the cache string will depend on 5.7 and 8.0 branch.
5. Switch from `ubuntu-18.04` to `ubuntu-20.04` VM images.

Author: inikep