Bug 40784: Fix var_p/nightly_torbrowser_incremental_from after #40737

rbm.conf

@@ -661,7 +661,8 @@ gpg_allow_expired_keys: 1
       },
       nightly_torbrowser_incremental_from => sub {
         my ($project, $options) = @_;
-        my $nightly_dir = project_config($project, 'basedir', $options) . '/nightly';
+        my $nightly_dir = project_config($project, 'basedir', $options) . '/' .
+                project_config($project, 'var/projectname', $options) . '/nightly';
         my $current_version = project_config($project, 'var/torbrowser_version', $options);
         use Path::Tiny;
         return [] unless -d $nightly_dir;