CI custom build improvements

To make CI debugging easier, the following parameters need to be introduced (or checked existing ones) for custom build:

• REGRESSION_TESTS - list of regression tests to be executed
• RUN_MTR, default True
• RUN_UPGRADE, default True
• RUN_MULTINODE_MTR, default False
• RUN_MULTINODE_UPGRADE, default False

check:

• EXTERNAL_PACKAGES_URL

current custom build document https://github.com/mariadb-corporation/mariadb-columnstore-engine/blob/develop/docs/custom-build.md