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Fixes #18070 GHC_STAGE is the stage of the compiler we're building, it should be 1,2(,3?). But make was generating 0 and 1. Hadrian does this correctly using a similar `+ 1`: https://gitlab.haskell.org/ghc/ghc/-/blob/eb8115a8c4cbc842b66798480fefc7ab64d31931/hadrian/src/Rules/Generate.hs#L245
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