Add job_name as a staticmethod in SLURMEnvironment class

[AndresAlgaba]

What does this PR do?

Fixes #10697, and moved the staticmethod job_id (out of the properties) for readability of the code.

Does your PR introduce any breaking changes? If yes, please list them.

None

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[AndresAlgaba]

Should any new tests be added? And should the changelog be updated for this refactoring?

[awaelchli]

Looks great! Adding a single line here (assert None):

https://github.com/PyTorchLightning/pytorch-lightning/blob/dca1776870e9dd7236c03d7ccd55fcb09a9bb5e7/tests/plugins/environments/test_slurm_environment.py#L24

and one here with a fictional value for a job name

https://github.com/PyTorchLightning/pytorch-lightning/blob/dca1776870e9dd7236c03d7ccd55fcb09a9bb5e7/tests/plugins/environments/test_slurm_environment.py#L52

should do it.