- ubuntu jobs are using the systems compiler and the Docker build image
  does not contain the GCC 6.4.0 package; CI scripts are modified to
  conditionally source the gcc environment setup script

- when running `make demo-cluster` on ubuntu, it looks like `su` does not
  preserve the environment set by "greenplum_path.sh" so we re-source
  "greenplum_path.sh" as gpadmin before starting the demo cluster
Co-authored-by: NJesse Zhang <jzhang@pivotal.io>
Co-authored-by: NBradford D. Boyle <bboyle@pivotal.io>