test: Don't add a channel for the force-no-daemon installer variant

Was probably an overlook of when the tests were first added, but that
now messes-up with the `nix-channel --update` that happens down the line
(and can't access the network since we're inside a Nix build)

tests/installer/default.nix

@@ -17,7 +17,7 @@ let
       script = ''
         tar -xf ./nix.tar.xz
         mv ./nix-* nix
-        ./nix/install --no-daemon
+        ./nix/install --no-daemon --no-channel-add
       '';
     };