Get rid of virtual Goal::init()

Now, each class provides the initial coroutine by value. This avoids
some sketchy virtual function stuff, and will also be further put to
good use in the next commit.

src/libstore/build/drv-output-substitution-goal.cc

@@ -12,7 +12,7 @@ DrvOutputSubstitutionGoal::DrvOutputSubstitutionGoal(
     Worker & worker,
     RepairFlag repair,
     std::optional<ContentAddress> ca)
-    : Goal(worker)
+    : Goal(worker, init())
     , id(id)
 {
     name = fmt("substitution of '%s'", id.to_string());