Use nix-shell --pure to avoid running externally installed tools

[noamraph]

I followed the original instructions, and was expecting to see bash: make: command not found, but instead found make to be installed.

I realized that it was the make from my own system. It seems to me better to not have the tools from my own system available. So I suggest adding --pure to the instructions, with an explanation of what it does.

[jtojnar]

--pure is quite annoying if you e.g. use nano to edit the files. And, in the past I had it break shell prompt IIRC (not sure what was responsible).

[noamraph]

@jtojnar I think that for educational reasons it's better to add --pure with an explanation, so if someone wants nano they'll know what to do. It seems to me like with --pure you're on the safe side, in an environment much similar to the build environment, and if you need your tools, you can always re-run without --pure. Also, it was just confusing to me to see that make is available, contrary to what's described, so I think it's better to avoid this confusion for other readers.

But obviously, you have much more experience that I have, so I would be fine with any decision.