Update GHC 9.12 and HEAD git pins #2797

Job	Run time
cabal	1s
build-extra-hackages	1s
cabal-doExactConfig	1s
check-materialization-concurrency	1s
check-path-support	1s
closure-size	1s
docs	1s
haskell-nix-roots-do-not-require-IFDs	1s
hix	1s
hix-cabal	1s
hydra-ifdLevel-0-and-1	1s
hydra-ifdLevel-2	1s
hydra-ifdLevel-3	1s
hydra-without-remote-builders-ghc8107	1s
hydra-without-remote-builders-ghc9101	1s
maintainer-scripts	1s
multi-target	1s
nix-build	1s
nix-tools	1s
plan-extra-hackages	1s
runghc	1s
shellFor-multiple-package	1s
shellFor-hoogle	1s
shellFor-not-depends	1s
shellFor-single-package	1s
tests-benchmarks	1s
unit-tests	1s
update-docs	1s
	28s

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