Hand a provisioned worker its token in the environment, not argv

The generated batch script exported nothing and passed `--token`, which
any user on a shared compute node reads from `ps`. It now exports
FLUKSIO_WORKER_TOKEN after `prerun`, which the worker already reads.

docs/code/workers.md

@@ -240,6 +240,9 @@ When a node needs a machine nothing attached can give, and a profile fits, the
 engine `sbatch`es one over the system `ssh`, and the run waits meanwhile, saying
 so. `prerun` owns the environment: a `module load`, or a venv with
 `fluksio-worker` already in it. The generated script runs no `pip install`.
+It hands the worker a token of its own, good for 24 hours, as
+`FLUKSIO_WORKER_TOKEN` rather than `--token`, so other users of a shared
+compute node cannot read it from `ps`.
 
 One outstanding request per profile, however often it is asked for. A job that
 never attaches within `provision_timeout_s` is `scancel`led, as is anything