Separate proxy build pipelines into two pipeline based on the build_target value

With the current logic, both builds will be triggered because the conditional that decides which proxy will be built was not defined in the pipeline rules. We just used a placeholder value proxy for the BUILD_TARGET variable. We were just checking if the variable was defined in the pipeline variables but did not use it to distinguish between which build would be triggered. As long as it was defined, the pipeline would run, but the catch was that it would run both pipelines.