Nix CI

`main`: add --json-schema / -j flag #4105

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Run time

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Job	Run time
nix-build (ubuntu-latest)	5m 50s
nix-eval (ubuntu-latest)	2m 25s
nix-build (macos-latest)	7m 14s
nix-eval (macos-latest)	4m 40s
	20m 9s