Add option to apply workspace concurrency code to custom workspaces

This change adds a checkbox to the custom workspace settings section to pass the workspace through the concurrency code. We have many jobs that use the same large Perforce repository, and to have a separate checkout of it for each job would use a large amount of disk space. This setup allows us to have one checkout for each executor with the jobs sharing the checkouts, which keeps the disk usage to an acceptable and constant level.