fix(herdi-mac): capture self weakly inside the concurrent task

The outer closures already captured self weakly, but the Task they start is a
separate concurrent context, and reading the outer capture from inside it is an
error as of Swift 5.10 -- which is what the macos-14 runner's default Xcode 15.4
compiles with. It builds locally only on 15.2 or older, where the same code is a
warning. Capturing weakly on the Task itself keeps the semantics identical.