If a build aborts, mark any remaining active build steps as aborted

See e.g. http://hydra.nixos.org/build/4915744.

P.S. existing active build steps of finished builds can be marked as
aborted by running:

update buildsteps set busy = 0, status = 4
  where (build, stepnr) in
    (select s.build, s.stepnr from buildsteps s join builds b on s.build = b.id where b.finished = 1 and s.busy = 1);

src/script/hydra-build

@@ -292,6 +292,9 @@ sub doBuild {
             addBuildProducts($db, $build);
         }
 
+        # Mark any remaining active build steps as aborted.
+        $build->buildsteps->search({ busy => 1 })->update({ busy => 0, status => 4, stoptime => time });
+
         $build->update(
             { finished => 1
             , busy => 0