Introduce NIX_INSTALLER_NO_CHANNEL_ADD which skips nix-channel --add

scripts/install-nix-from-closure.sh

@@ -130,6 +130,7 @@ if [ -z "$NIX_SSL_CERT_FILE" ] || ! [ -f "$NIX_SSL_CERT_FILE" ]; then
 fi
 
 # Subscribe the user to the Nixpkgs channel and fetch it.
+if [ -z "$NIX_INSTALLER_NO_CHANNEL_ADD" ]; then
     if ! $nix/bin/nix-channel --list | grep -q "^nixpkgs "; then
         $nix/bin/nix-channel --add https://nixos.org/channels/nixpkgs-unstable
     fi
@@ -139,6 +140,7 @@ if [ -z "$_NIX_INSTALLER_TEST" ]; then
             echo "To try again later, run \"nix-channel --update nixpkgs\"."
         fi
     fi
+fi
 
 added=
 p=$HOME/.nix-profile/etc/profile.d/nix.sh