Tests

readme: update nix instructions (#181) #1370

Job	Run time
x64 (ubuntu-18.04)	1s
x64 (ubuntu-20.04)	2m 27s
x64 (macos-10.15, https://github.com/neovim/neovim/releases/download/nightly/nvim-macos.tar.gz)	1s
	2m 29s

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