Fix on_change redefinition warning in survey callback handling

[blaipr]

Summary

• Fix flake8 F811 (redefinition of unused name) in job_template.py and workflow_job_template.py
• The on_change callback was first assigned None, then conditionally redefined as a nested function, triggering F811
• Refactored to define the callback function unconditionally at the top, then assign it to a survey_callback variable only when needed

Test plan

• flake8 --select=F811 plugins/modules/job_template.py plugins/modules/workflow_job_template.py returns no errors
• Survey spec updates still work correctly on job templates and workflow job templates