Invalid syntax in bootstrap.py

Currently line 341 of boostrap.py which is:

f"{'public' if public else 'private'}/{'stable' if stable else 'beta'}"

Triggers invalid syntax on all build runs of the beat packages.

This is valid Python 3.6 code however, until all CI Linux runners are updated to Buster, it cannot be used.

The bootstrap process runs using the system Python which is currently Python 3.5 on the Linux nodes.