Fix selection of crossPlaforms for cross compiltion #3064

Job	Run time
nix-build	4m 34s
cabal	1m 2s
runghc	1m 0s
unit-tests	6s
cabal-doExactConfig	1m 7s
maintainer-scripts	10s
multi-target	2m 41s
plan-extra-hackages	34s
shellFor-hoogle	1m 40s
shellFor-multiple-package	2m 35s
shellFor-not-depends	21s
shellFor-single-package	1m 45s
tests-benchmarks	14s
build-extra-hackages	13s
check-materialization-concurrency	20s
check-path-support	19s
closure-size	8s
docs	13m 41s
haskell-nix-roots-do-not-require-IFDs	1m 20s
hix	2m 46s
hydra-ifdLevel-0-and-1	11m 29s
hydra-ifdLevel-2	10m 16s
hydra-ifdLevel-3	30m 15s
hydra-without-remote-builders-ghc9101	4m 24s
hydra-without-remote-builders-ghc967	3m 37s
update-docs	10s
hix-cabal	31s
nix-tools	1m 40s
	1h 38m 58s

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