Fix building ghc964 with latest nixpkgs-unstable pin #2996

Job	Run time
unit-tests	41s
runghc	43s
cabal	40s
multi-target	7m 31s
nix-build	1h 10m 35s
shellFor-hoogle	6m 55s
cabal-doExactConfig	38s
shellFor-multiple-package	27s
tests-benchmarks	10s
shellFor-single-package	6m 24s
build-extra-hackages	36s
closure-size	5s
docs	19m 19s
hydra-ifdLevel-0-and-1	14m 15s
hydra-ifdLevel-2	20m 10s
hydra-ifdLevel-3	30m 14s
hix	12m 15s
maintainer-scripts	41s
plan-extra-hackages	14s
shellFor-not-depends	42s
update-docs	14s
check-materialization-concurrency	30s
check-path-support	41s
haskell-nix-roots-do-not-require-IFDs	1m 23s
hix-cabal	15m 8s
hydra-without-remote-builders-ghc9101	11m 15s
hydra-without-remote-builders-ghc966	17s
nix-tools	52s
	3h 43m 35s

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