`-ddump-hi-diffs` should report more useful information with flag changing

Motivation

I am trying to implement a GitHub Actions step that is expensive to build (90 minutes). When I use the standard cache action, GHC sees the old files but recompiles everything due to [flags changed]. I am not changing any flags between runs, so something is weird here. I want to understand what flags changed, so I pass -ddump-hi-diffs. The resulting output is useless:

The code here only checks the hash: https://gitlab.haskell.org/ghc/ghc/-/blob/wip/fromListN/compiler/GHC/Iface/Recomp.hs?ref_type=heads#L356-367

I am completely unable to diagnose what flags actually changed!

Proposal

We should store the actual flags we're fingerprinting in the interface file, and fingerprintDynFlags should return (Fingerprint, InterfaceDynFlags). When this case gets tripped, we should diffInterfaceDynFlags :: InterfaceDynFlags -> InterfaceDynFlags -> [String] and render out exactly what changed.