Fix bsp clean

[lihaoyi]

Fixes #5448

Task.Anons cannot call Task.Commands, so we move the clean logic into a separate def cleanTask: Task.Anon that can be shared between both

[lefou]

Task.Anons cannot call Task.Commands

I think they can, but if the commands are exclusive = true, it won't work. This is the same issue I hit in #5424

While working around might help, we eventually need to properly handle that case.