The "fetch-upstream-resources" script also updated the toc.yaml,
based on values in the _config.yml.
As a result, any change in either the "_config.yml" or "toc.yaml"
would also result in the upstream resources to be fetched again.
This patch separates the step to update the toc, so that the
upstream resources can be cached. This _does_ mean that remote
sources can get outdated (only will be rebuilt when changing
ENGINE_BRANCH or DISTRIBUTION_BRANCH). That should not be a
problem for actual deployments, which don't use caching.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>