Bump nixpkgs pins #898

Job	Run time
cabal-doExactConfig	1m 10s
build-extra-hackages	33s
check-materialization-concurrency	29s
cabal	22s
check-path-support	5m 37s
closure-size	23s
docs	12m 59s
haskell-nix-roots-do-not-require-IFDs	51s
hix	2h 30m 40s
hix-cabal	1m 33s
hydra-ifdLevel-0-and-1	1m 11s
hydra-ifdLevel-3-and-ghc-8-10	43s
hydra-ifdLevel-2	43s
hydra-ifdLevel-3-and-ghc-9-2	29s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	45s
maintainer-scripts	33s
multi-target	3m 36s
hydra-without-remote-builders	1m 30s
nix-build	1m 55s
nix-tools-build	6m 16s
plan-extra-hackages	36s
runghc	43s
shellFor-hoogle	41s
shellFor-multiple-package	1m 46s
shellFor-not-depends	1m 17s
shellFor-single-package	52s
tests-benchmarks	39s
unit-tests	23s
update-docs	12s
	3h 19m 27s

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