Portable Python job name hard-coded to "job"

See [1]. `job_name` is already taken by Google Cloud options [2], so I guess we should create a new option (maybe `portable_job_name` to avoid disruption).

[[1] https://github.com/apache/beam/blob/55588e91ed8e3e25bb661a6202c31e99297e0e79/sdks/python/apache_beam/runners/portability/portable_runner.py#L294|https://github.com/apache/beam/blob/55588e91ed8e3e25bb661a6202c31e99297e0e79/sdks/python/apache_beam/runners/portability/portable_runner.py#L294]

[2] [https://github.com/apache/beam/blob/c5bbb51014f7506a2651d6070f27fb3c3dc0da8f/sdks/python/apache_beam/options/pipeline_options.py#L438](https://github.com/apache/beam/blob/c5bbb51014f7506a2651d6070f27fb3c3dc0da8f/sdks/python/apache_beam/options/pipeline_options.py#L438)

Imported from Jira [BEAM-8348](https://issues.apache.org/jira/browse/BEAM-8348). Original Jira may contain additional context.
Reported by: ibzib.