Don't ever try to install Nix on NixOS

scripts/setup

@@ -19,7 +19,10 @@ exit_unless_os_supported
 ####
 setup_header "Checking prerequisites..."
 
-if ! program_exists "curl"; then
+_need_curl=0
+! is_nixos && ! program_exists nix && _need_curl=1
+
+if _need_curl && ! program_exists "curl"; then
   cecho "@b@yellow[[Please install curl before running setup.]]"
   exit 1
 fi
@@ -45,7 +48,7 @@ fi
 ####
 setup_header "Installing requirements..."
 
-if ! program_exists nix; then
+if [ "$IN_NIX_SHELL" != 'pure' ] && ! is_nixos && ! program_exists nix; then
   required_version=$(toolversion nix)
   NIX_INSTALLER_NO_MODIFY_PROFILE=1 bash <(curl https://nixos.org/releases/nix/nix-${required_version}/install) --no-daemon
   if [ $? -eq 0 ]; then