CI enforces `-Wunused-local-binds`, but default build flavor does not
Motivation
I had a CI fail because I left an unused local binding in my code (see https://gitlab.haskell.org/ghc/ghc/-/jobs/1005507#L892). My source tree had passed ./hadrian/build test
locally.
Proposal
I propose that -Wunused-local-binds
be enabled for all build flavors.
Note: this ticket is very similar to #21273