Move runPostBuildHook out of DerivationBuilder

It is suppposed to be "post build" not "during the build" after all. Its
location now matches that for the hook case (see elsewhere in
`DerivationdBuildingGoal`).

It was in a try-catch before, and now it isn't, but I believe that it is
impossible for it to throw `BuildError`, which is sufficient for this
code motion to be correct.

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

@@ -506,11 +506,6 @@ std::variant<std::pair<BuildResult::Status, Error>, SingleDrvOutputs> Derivation
            being valid. */
         auto builtOutputs = registerOutputs();
 
-        StorePathSet outputPaths;
-        for (auto & [_, output] : builtOutputs)
-            outputPaths.insert(output.outPath);
-        runPostBuildHook(store, *logger, drvPath, outputPaths);
-
         /* Delete unused redirected outputs (when doing hash rewriting). */
         for (auto & i : redirectedOutputs)
             deletePath(store.Store::toRealPath(i.second));