Prevent overriding virtual methods that are called in a destructor

Virtual methods are no longer valid once the derived destructor has
run. This means the compiler is free to optimize them to be
non-virtual.

Found using clang-tidy

src/libmain/progress-bar.cc

@@ -108,7 +108,8 @@ public:
         stop();
     }
 
-    void stop() override
+    /* Called by destructor, can't be overridden */
+    void stop() override final
     {
         {
             auto state(state_.lock());

src/libstore/build/local-derivation-goal.hh

@@ -272,8 +272,10 @@ struct LocalDerivationGoal : public DerivationGoal
 
     /**
      * Forcibly kill the child process, if any.
+     *
+     * Called by destructor, can't be overridden
      */
-    void killChild() override;
+    void killChild() override final;
 
     /**
      * Kill any processes running under the build user UID or in the

src/libstore/build/substitution-goal.hh

@@ -114,7 +114,8 @@ public:
     void handleChildOutput(int fd, std::string_view data) override;
     void handleEOF(int fd) override;
 
-    void cleanup() override;
+    /* Called by destructor, can't be overridden */
+    void cleanup() override final;
 
     JobCategory jobCategory() override { return JobCategory::Substitution; };
 };