Install script adds duplicate keys

[asymmetric]

My nix.conf already contained a substituers key, and the install script added another one at the bottom of the script.

We should instead append to the existing key.

[nanexcool]

Yes, great idea! We should discuss this. What we add in nix.conf is basically what you get by installing cachix then running cachix use dapp. But... I noticed on Macs sometimes cachix installs from source and compiles and takes a long time.

This is why we only modify the nix.conf file naively. Since it's 3 lines (with 1 comment) we could check for the existance of these lines and only add if not found.