Fixes Trac #14470.
It looks like switching to the ghc-diffs repo you created as a "staging" one for the Circle CI builds did the trick. Not sure how important the core detection logic was in making this work, but I suppose it is anyway better not to just blindly run with -j9.
I triggered a build from the phabricator interface, which resulted in this successful Circle CI build: https://circleci.com/gh/ghc/ghc-diffs/11
The Phab side is here: https://phabricator.haskell.org/harbormaster/build/48987/ -- it is not yet rich in information about the build, but this is also the very first build that succeeds in this setup.