nixos/sshServe: use bash as default shell for nix-ssh user

Using the user-set default shell (which is intended for non-system users) for the nix-ssh user can lead to unpredictable behavior, such as `fish` complaining about the unwritable home directory on every connection. Bash is guaranteed to be available and work as expected, so explicitly use it instead.

nixos/modules/services/misc/nix-ssh-serve.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 let cfg = config.nix.sshServe;
@@ -46,7 +46,7 @@ in {
       description = "Nix SSH store user";
       isSystemUser = true;
       group = "nix-ssh";
-      useDefaultShell = true;
+      shell = pkgs.bashInteractive;
     };
     users.groups.nix-ssh = {};