[Backend] Add start evaluation worker function call in challenge creation using github API

### Description

When a challenge is created using github based challenge creation pipeline, follow the steps mentioned [here](https://github.com/Cloud-CV/EvalAI-starters#create-challenge-using-github) to use the set up, we don't automatically start evaluation workers for the challenge. We should make a `start_worker` celery task call when a new challenge is created using github based set up. [Here's](https://github.com/Cloud-CV/EvalAI/blob/master/apps/challenges/views.py#L1472) the reference to how we are doing it for challenge creation using zip file.

### Deliverable

- [x] Backend API changes to trigger `start_workers` celery task when challenge is created using github based setup.

Start in apps/challenges/views.py around line 1472 and compare the existing zip-based challenge creation flow with the GitHub-based setup described in EvalAI-starters. Verify where challenge creation completes and confirm that the start_workers Celery task is triggered for GitHub-created challenges.

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