Since neither a build tree, nor the actual tree is returned from
`build_stages` the short circuit code that checks if the tree is
already present in the store, can be moved before the build tree
retrival. As a result, the short-circuit check in `Pipeline.run`
is now redundant. It was there to make sure that if we have the
tree associated with a pipeline, its build pipeline would also
not be needed. With the short-circuit now happening before the
access of the build pipeline in `build_stages` this is ensured.