Fix Emacs color inside tmux #670

Job	Run time
pkgs-linux: nixpkgs: build	1m 52s
pkgs-linux: nixpkgs: prebuilt	4m 21s
pkgs-linux: haskell-nix: build	51s
pkgs-linux: haskell-nix: prebuilt	52s
pkgs-linux: shajra: build	1m 55s
pkgs-linux: shajra: prebuilt	59s
installers-linux	16s
pkgs-darwin: nixpkgs: build	3m 47s
pkgs-darwin: nixpkgs: prebuilt	7m 6s
pkgs-darwin: haskell-nix: build	1m 55s
pkgs-darwin: haskell-nix: prebuilt	1m 54s
pkgs-darwin: shajra: build	1m 59s
pkgs-darwin: shajra: prebuilt	5m 23s
	33m 10s

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