The upload-to workflows for *-backports and experimental lack default task-configuration

Due to various limitations, there's a set of workflows (the upload-to-* for experimental and {trixie,bookworm}-backports{,-sloppy}) that rely on a task configuration named "overlay-not-automatic" instead of the "default" task configuration.

And we currently can't configure the "overlay-not-automatic" task configuration to inherit from the "default" task configuration.

This means that some custom configuration (like an hypothetical option to plan some tasks on larger workers) will not be used for uploads to experimental or backports which is not ideal.