In DerivationBuildingGoal Demote actLock to local variable

It doesn't need to be a field any more, because we just use it with two
loops.

src/libstore/build/derivation-building-goal.cc

@@ -470,6 +470,11 @@ void DerivationBuildingGoal::started()
 
 Goal::Co DerivationBuildingGoal::tryToBuild()
 {
+    /**
+     * Activity that denotes waiting for a lock.
+     */
+    std::unique_ptr<Activity> actLock;
+
     bool useHook;
 
     while (true) {

src/libstore/include/nix/store/build/derivation-building-goal.hh

@@ -92,11 +92,6 @@ struct DerivationBuildingGoal : public Goal
 
     std::unique_ptr<Activity> act;
 
-    /**
-     * Activity that denotes waiting for a lock.
-     */
-    std::unique_ptr<Activity> actLock;
-
     std::map<ActivityId, Activity> builderActivities;
 
     /**