Fix VM environment variables #703

Job	Run time
pkgs-linux: nixpkgs: build	2m 47s
pkgs-linux: nixpkgs: prebuilt	6m 21s
pkgs-linux: haskell-nix: build	2m 11s
pkgs-linux: haskell-nix: prebuilt	1m 58s
pkgs-linux: shajra: build	2m 33s
pkgs-linux: shajra: prebuilt	2m 0s
installers-linux	21s
pkgs-darwin: nixpkgs: build	5m 32s
pkgs-darwin: nixpkgs: prebuilt	13m 26s
pkgs-darwin: haskell-nix: build	3m 45s
pkgs-darwin: haskell-nix: prebuilt	3m 17s
pkgs-darwin: shajra: build	3m 23s
pkgs-darwin: shajra: prebuilt	4m 15s
	51m 49s

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