Previously, WorkspaceBuildBuilder.doInTX() inserted provisioner jobs
with empty tags and used a loop in AcquireProvisionerJob that could
match other tests' pending jobs when parallel tests share a database.
Add a unique tag (jobID -> "true") to each provisioner job at insert
time, then use that tag in AcquireProvisionerJob to target only the
correct job. This follows the same pattern used in dbgen.ProvisionerJob.
Closescoder/internal#1367