GitHub: T6494: add workflow to build custom ISO for integration tests

For every PR we will build the vyos-1x package and include it in a custom build
of the VyOS ISO image (generic flavor) in order to execute the commonly
available smoketests:

• make test (CLI smoketests)
• make testc (config load & migration tests)
• make testraid (Installation of RAID1 system with disk replacement)

This is a measure agains merged PRs that break the smoketests in the end.