Clean up temp roots in a more C++ way
nix-daemon: Call exit(), not _exit()

This was preventing destructors from running. In particular, it was
preventing the deletion of the temproot file for each worker
process. It may also have been responsible for the excessive WAL
growth on Hydra (due to the SQLite database not being closed
properly).

Apparently broken by accident in
8e9140cfdef9dbd1eb61e4c75c91d452ab5e4a74.