Regression in non-MT mode: repeated current directory allocation in TaskEnvironment

A regression was observed in non-MT mode where migrated tasks use `TaskEnvironment` and it repeatedly allocates the current directory during path absolutization. This leads to unnecessary overhead. Suggested fix: lazily cache the directory once per task. If the task uses `TaskEnvironment`, then it is MT aware. It should not thus change the working directory, so there is no need to request it each time for path absolutization.