Include all pre-existing ghc pkgs in shell, unless exactDeps #3128

Job	Run time
cabal	1m 7s
unit-tests	22s
nix-build	3m 18s
build-extra-hackages	54s
hix	1m 16s
maintainer-scripts	22s
cabal-doExactConfig	1m 3s
multi-target	1m 30s
plan-extra-hackages	20s
runghc	32s
shellFor-hoogle	23s
shellFor-multiple-package	22s
shellFor-not-depends	20s
shellFor-single-package	22s
tests-benchmarks	9s
check-materialization-concurrency	19s
check-path-support	10s
closure-size	3s
docs	13m 44s
haskell-nix-roots-do-not-require-IFDs	1m 8s
hix-cabal	20s
hydra-ifdLevel-0-and-1	4m 37s
hydra-ifdLevel-2	15m 12s
hydra-ifdLevel-3	30m 17s
hydra-without-remote-builders-ghc9102	3m 38s
hydra-without-remote-builders-ghc967	3m 49s
nix-tools	16s
update-docs	8s
	1h 26m 1s

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