LifecycleService should turn on part_builld_subdir for future bases (e.g. ubuntu 27.04+)

[lengau]

What needs to get done

Once this PR is merged: canonical/craft-parts#1652

For future bases (Ubuntu 27.04+), we will configure the lifecycle manager to use the build subdir rather than the build directory.

Why it needs to get done

This is a load-bearing bug in craft parts that we should eliminate in the long term.