Add pytest_custom_ipv8.yml

[drew2a]

This PR closes #6938 by adding the pytest_custom_ipv8.yml workflow that can be run manually in the same way that the ubuntu/window/macOS build does.

This workflow has the following arguments:

      python-version:
        description: Python version
        default: '3.8'
        type: string
        required: true

      ipv8-git-ref:
        description: IPv8 Git Ref
        default: 'master'
        type: string
        required: true

This PR can be tested appropriately after merging.

Ref:

• https://pip.pypa.io/en/stable/topics/vcs-support/

[qstokkink]

praise: Perfect in every way; very much approved.

[drew2a]

You can try the workflow here: https://github.com/Tribler/tribler/actions/workflows/pytest_custom_ipv8.yml