Move killChild call from ~DerivationBuildingGoal to ~DerivationBuilder

Sadly we cannot unexpose `DerivationBuilder::killChild` yet, because
`DerivationBuildingGoal` calls it elsewhere, but we can at least haave a
better division of labor between the two destructors.

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

@@ -53,11 +53,6 @@ DerivationBuildingGoal::~DerivationBuildingGoal()
 {
     /* Careful: we should never ever throw an exception from a
        destructor. */
-    try {
-        killChild();
-    } catch (...) {
-        ignoreExceptionInDestructor();
-    }
 #ifndef _WIN32 // TODO enable `DerivationBuilder` on Windows
     if (builder)
         builder.reset();

src/libstore/unix/build/derivation-builder.cc

@@ -94,6 +94,11 @@ public:
     {
         /* Careful: we should never ever throw an exception from a
            destructor. */
+        try {
+            killChild();
+        } catch (...) {
+            ignoreExceptionInDestructor();
+        }
         try {
             stopDaemon();
         } catch (...) {