Right now, the build of the unstable modules added in #15442 copies the L1s of the unstable modules alongside everything else.

However, those L1s are already present in aws-cdk-lib (yes, those get copied even for unstable modules; only the L2s get stripped from there. This was added in #14135). Given that, we probably shouldn't copy the L1s again to the separately-released unstable modules.