Move derivationType from DerivationGoal to LocalDerivationGoal

The super class doesn't actually care.

src/libstore/build/derivation-goal.cc

@@ -533,9 +533,6 @@ Goal::Co DerivationGoal::gaveUpOnSubstitution()
 
     debug("added input paths %s", worker.store.showPaths(inputPaths));
 
-    /* What type of derivation are we building? */
-    derivationType = drv->type();
-
     /* Okay, try to build.  Note that here we don't wait for a build
        slot to become available, since we don't need one if there is a
        build hook. */

src/libstore/build/derivation-goal.hh

@@ -198,11 +198,6 @@ struct DerivationGoal : public Goal
     std::unique_ptr<HookInstance> hook;
 #endif
 
-    /**
-     * The sort of derivation we are building.
-     */
-    std::optional<DerivationType> derivationType;
-
     BuildMode buildMode;
 
     std::unique_ptr<MaintainCount<uint64_t>> mcExpectedBuilds, mcRunningBuilds;

src/libstore/unix/build/local-derivation-goal.cc

@@ -150,6 +150,13 @@ struct LocalDerivationGoal : DerivationGoal, RestrictionContext
      */
     std::shared_ptr<AutoDelete> autoDelChroot;
 
+    /**
+     * The sort of derivation we are building.
+     *
+     * Just a cached value, can be recomputed from `drv`.
+     */
+    std::optional<DerivationType> derivationType;
+
     /**
      * Stuff we need to pass to initChild().
      */
@@ -478,7 +485,8 @@ Goal::Co LocalDerivationGoal::tryLocalBuild()
         co_return tryToBuild();
     }
 
-    assert(derivationType);
+    /* Cache this */
+    derivationType = drv->type();
 
     /* Are we doing a chroot build? */
     {