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.

I propose that -Wunused-local-binds be enabled for all build flavors.

Note: this ticket is very similar to #21273

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