Support repairPath on most stores.

More progress on issue #5729

The method trivially generalizes to be store-implementation-agnostic, in
fact.

However, we force it to continue to be unimplemented with `RemoteStore`
and `LegacySSHStore` because the implementation we'd get via the
generalization is probably not the one users expect. This keeps our
hands untied to do it right going forward.

For more about the tension between the scheduler logic being
store-type-agnostic and remote stores doing their own scheduling, see
issues #5025 and #5056.

src/libstore/build/entry-points.cc

@@ -119,7 +119,7 @@ void Store::ensurePath(const StorePath & path)
 }
 
 
-void LocalStore::repairPath(const StorePath & path)
+void Store::repairPath(const StorePath & path)
 {
     Worker worker(*this, *this);
     GoalPtr goal = worker.makePathSubstitutionGoal(path, Repair);