Avoid referencing source dir after it may be removed.

This "last-build-rev" persistent option is probably more appropriate for
Module.pm anyways, since there is already a similar "last-install-rev"
check in there.

modules/ksb/Application.pm

@@ -1383,7 +1383,6 @@ sub _buildSingleModule
     $$startTimeRef = time;
     if ($module->build())
     {
-        $module->setPersistentOption('last-build-rev', $module->currentScmRevision());
         $fail_count = 0;
     }
     else {

modules/ksb/Module.pm

@@ -425,6 +425,8 @@ sub build
         return 0;
     }
 
+    $self->setPersistentOption('last-build-rev', $self->currentScmRevision());
+
     # TODO: This should be a simple phase to run.
     if ($self->getOption('run-tests'))
     {