Add mention of the Coq Nix Toolbox. #1773

Job	Run time
Nix (ubuntu-latest)	3m 14s
client-compile	19s
Nix (macOS-latest)	7m 18s
build (ubuntu-latest, 4.14.x)	16m 26s
build (ubuntu-latest, 4.13.x)	16m 37s
build (ubuntu-latest, 4.12.x)	18m 0s
Nix Flake Check	1m 9s
build (ubuntu-latest, 4.11.x)	18m 23s
build (ubuntu-latest, 5.0.x)	16m 29s
build (macos-latest, 4.14.x)	26m 7s
build (Windows Latest, 4.14, windows-latest)	23m 56s
	2h 27m 58s

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