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.

backend/tests/flow/test_provision.py

@@ -87,6 +87,9 @@ def test_the_script_starts_a_worker_that_dials_back_and_stops_when_done(monkeypa
     assert "--labels gpu" in script
     # Without this the job holds its allocation until the walltime.
     assert "--max-idle 120" in script
+    # The token travels in the environment: argv is in `ps` for anyone.
+    assert "--token" not in script
+    assert "\nexport FLUKSIO_WORKER_TOKEN=ey" in script
 
 
 def test_one_job_per_profile_however_often_it_is_asked_for(monkeypatch):