Revamp nix flake for nix-tools #892

Job	Run time
unit-tests	18s
cabal	25s
runghc	16s
build-extra-hackages	9s
check-materialization-concurrency	19s
cabal-doExactConfig	10s
check-path-support	17s
closure-size	5s
docs	10m 20s
hix	33m 44s
hydra-ifdLevel-0-and-1	9s
hydra-ifdLevel-2	6s
hydra-ifdLevel-3-and-ghc-8-10	6s
hydra-ifdLevel-3-and-ghc-9-2	6s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	7s
maintainer-scripts	9s
multi-target	41s
nix-build	3m 27s
plan-extra-hackages	9s
shellFor-hoogle	10s
shellFor-multiple-package	12s
shellFor-not-depends	20s
shellFor-single-package	16s
tests-benchmarks	11s
update-docs	5s
haskell-nix-roots-do-not-require-IFDs	8s
hix-cabal	2m 58s
hydra-without-remote-builders	1m 23s
nix-tools-build	5s
	56m 51s

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