Fix "supportHpack: command not found" (#2007) #859

Job	Run time
unit-tests	9s
cabal	13s
cabal-doExactConfig	16s
nix-build	1m 49s
runghc	26s
shellFor-multiple-package	19s
multi-target	21s
tests-benchmarks	9s
shellFor-single-package	45s
docs	9m 46s
build-extra-hackages	11s
hix	1m 27s
hydra-ifdLevel-0-and-1	10s
hydra-ifdLevel-2	22s
maintainer-scripts	20s
plan-extra-hackages	7s
shellFor-hoogle	9s
hydra-ifdLevel-3-and-ghc-8-10	20s
shellFor-not-depends	24s
check-materialization-concurrency	23s
check-path-support	8s
closure-size	5s
haskell-nix-roots-do-not-require-IFDs	7s
hix-cabal	1m 19s
hydra-ifdLevel-3-and-ghc-9-2	7s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	20s
hydra-without-remote-builders	4m 50s
nix-tools-build	9m 55s
update-docs	3s
	35m 0s

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