replace build-flow-plugin by workflow-aggregator
replace build-flow-plugin by workflow-aggregator

The build-flow-plugin is discontinued and has known vulnerabilities
(see https://wiki.jenkins.io/display/JENKINS/Build+Flow+Plugin). So
follow the recommended successor plugin.

Change-Id: I3b38425ff40ff572c1c2d932f43daf196a3d3be2

bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration