When using Nix on NixOS, only root can add binary caches to the system. This will force try-reflex to rebuild GHCJS from scratch, which takes hours. To use the binary cache, you can add the following lines to your /etc/nixos/configuration.nix:
nix.settings.substituters = [ "https://nixcache.reflex-frp.org" ];
nix.settings.trusted-public-keys = [ "ryantrinkle.com-1:JJiAKaRv9mWgpVAz8dwewnZe0AzzEAzPkagE9SP5NWI=" ];
If you already have one of these variables set up, just add these values to the existing lists.
Once it's been added, run sudo nixos-rebuild switch to make the change take effect, then run ./try-reflex as normal.
Note: If you'd prefer not to use nixcache.reflex-frp.org by default on your system, you can add it to nix.trusted-substituters instead of nix.substituters. This way, scripts like try-reflex will be allowed to use it, but other nix commands will ignore it. Once it's in trusted-substituters, you can always pass --option extra-binary-caches https://nixcache.reflex-frp.org to nix commands such as nix-build and nix-shell manually if you'd like to use it for a particular build.
Note: If the binary cache does not appear to be applying and you are seeing messages like
warning: substituter 'https://nixcache.reflex-frp.org' does not have a valid signature for path '/nix/store/...'
check (and consider removing or modifying) $HOME/.config/nix/nix.conf, as well.