manifest: add fedora-candidate-compose to list of repos

Late in the Fedora major release process adding this repo will
allow us to pull in candidate RPMs that are proposed to fix
release blocking or freeze exception issues. We will be able to
test out these proposed fixes sooner and also be more prepared
for releasing our final `next` release once the release is
deemed "Go".

See coreos#2705

manifest.yaml

@@ -11,6 +11,7 @@ repos:
   # GitHub Action.
   - fedora-next
   - fedora-next-updates
+  - fedora-candidate-compose
 
 include: manifests/fedora-coreos.yaml