Allow overriding the environment in the piuparts workflow

This is typically required to select a suitably recent piuparts. Document this requirement in the piuparts task.

We provided an environment parameter to the piuparts workflow, instead of another codename, as there are use-cases where it would make sense to have a fully custom lookup to locate an environment with a specific build of piuparts.

Carry these overrides forward to the qa and debian-pipeline workflows.

Fixes: #638 (closed) (the implementation part, workflows will need to be updated to set the override appropriately)