Use proper GHC option handling for passing multi-repl flags

[mpickering]

This refactoring modifies the logic to start the multi-repl to use runGHCWithResponseFile rather than directly constructing a command line.

Towards #10881

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[alt-romes]

Looks good!

[mpickering]

Thanks for the review Brandon and Rodrigo.

[ulysses4ever]

@geekosaur check it out: https://github.com/haskell/cabal/actions/runs/15887198438/job/44801969759?pr=10995

[geekosaur]

Yes, and I noticed a few minutes ago that cabal-plan here showed the old version of splitmix until I ran cabal update and cabal build all --dry-run locally. I think the new broken-on-Macs version got uploaded very recently (within the past week, and potentially today).

[geekosaur]

Anyway if you rebase on master it should work. But: it's a transitive dependency, and this breakage looks to be a PVP violation so slipped past everyone.

The good news is, it's not part of any of the released packages, it's part of the tests.

[ulysses4ever]

@geekosaur we have the index-state pinned (your PR that bumps it and runs into the splitmix issue isn't merged yet). So, how would you explain the failure here — with the old index-state?

[geekosaur]

cabal.validate.project isn't pinned, so it picked up the new version of splitmix as soon as Hackage was updated. Only the bootstrap and release project files are pinned.

[ulysses4ever]

Oh, my bad. I'm curious now what we were thinking when decided to pin one but not the other...

[geekosaur]

The idea was that we would find out about transitive deps that broke things before we moved the pin as part of a release. Which fails when someone breaks us while we're making a release....

[geekosaur]

Speaking of which, I need to move the patchup to where it'll be caught by the other project files: I bet bootstrap on Macs is now broken.