Remove buildUser from DerivationBuilder

The use of a `buildUser` is an implementation detail of some types of
sandboxes that shouldn't exposed.

src/libstore/build/derivation-goal.cc

@@ -895,8 +895,8 @@ Goal::Co DerivationGoal::tryToBuild()
         builder->startBuilder();
 
     } catch (BuildError & e) {
+        builder.reset();
         outputLocks.unlock();
-        builder->buildUser.reset();
         worker.permanentFailure = true;
         co_return done(BuildResult::InputRejected, {}, std::move(e));
     }

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

@@ -113,6 +113,11 @@ public:
 
 private:
 
+    /**
+     * User selected for running the builder.
+     */
+    std::unique_ptr<UserLock> buildUser;
+
     /**
      * The cgroup of the builder, if any.
      */
@@ -271,7 +276,7 @@ public:
     /**
      * Start building a derivation.
      */
-    void startBuilder() override;;
+    void startBuilder() override;
 
     /**
      * Tear down build environment after the builder exits (either on

src/libstore/unix/include/nix/store/build/derivation-builder.hh

@@ -145,11 +145,6 @@ struct DerivationBuilderCallbacks
  */
 struct DerivationBuilder : RestrictionContext
 {
-    /**
-     * User selected for running the builder.
-     */
-    std::unique_ptr<UserLock> buildUser;
-
     /**
      * The process ID of the builder.
      */