Use Agent.BuildDirectory instead of Agent.WorkFolder

[grotkiv]

Agent.WorkFolder is not mounted as docker volume by Azure Pipelines, if
containers are used. This causes the SynopsisDetectTask to fail because
the detect folder cannot be created.

This commit changes the WorkingDirectory to Agent.BuildDirectory. The
detect folder can be created and the detect script can be downloaded
properly even from a container.

[grotkiv]

Unfortunately, I'm not able to test this patch in our environment properly. Because the patch is just using another azure variable, I assumed it would be fine. As a quick fix this patch might be sufficient and helps others to get their container jobs running again. But the correct solution might be to use the Agent.ToolDirectory to download/install the detect.sh script.

In the meanwhile my workaround is to create a local directory (e.g. /home/build/synopsisDetect) on my build agents and mount this path as volume for my container job:

- job: Blackduck
  container:
    image: some-image:latest
    volumes:
    - /home/build/synopsisDetect:$(Agent.WorkFolder)/detect

Ensure the folder is writeable by the build user from within the container. This way the detect folder already exists and the detect script can be stored properly.