ci: build.sh: Drop changing working directory to CI_CONT_DIR

ci: build.sh: Drop changing working directory to CI_CONT_DIR

Firstly, this would mangle with "sourcing" this file in either
execution environment later down the road. Secondly, we won't need this
as future ci/helper patches will generate a throwaway script that will
take care of a correct execution of a build job in a similar fashion as
if the job ran in a GitLab environment.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>