Fix job_driver to use config for dev_source_dirs instead of os.environ.get hack

[robnagler]

The current code in job_driver/init.py uses os.environ.get("SIREPO_PKCLI_JOB_AGENT_DEV_SOURCE_DIRS", str(pkconfig.in_dev_mode())) to pass the dev_source_dirs flag to agents. This bypasses pkconfig and is fragile.

Replace with a proper pkconfig entry sirepo.job_driver.dev_source_dirs (bool, default pkconfig.in_dev_mode()) and use that value to set SIREPO_PKCLI_JOB_AGENT_DEV_SOURCE_DIRS in the agent env. No backwards compatibility needed -- update etc/run*.sh as required.