nfs-utils: set /etc/krb5.keytab as default path for rpc-gssd

Currently the `rpc-gssd.service` has a `ConditionPathExists` clause that can
never be met, because it's looking for stateful data inside `/nix/store`.

`auth-rpcgss-module.service` also only starts if this file exists.

Fixes NixOS/nixpkgs#29509.

nixos/modules/tasks/filesystems/nfs.nix

@@ -85,8 +85,14 @@ in
         enable = mkDefault false;
       };
 
+    systemd.services.auth-rpcgss-module =
+      {
+        unitConfig.ConditionPathExists = [ "" "/etc/krb5.keytab" ];
+      };
+
     systemd.services.rpc-gssd =
       { restartTriggers = [ nfsConfFile ];
+        unitConfig.ConditionPathExists = [ "" "/etc/krb5.keytab" ];
       };
 
     systemd.services.rpc-statd =

pkgs/os-specific/linux/nfs-utils/default.nix

@@ -39,7 +39,7 @@ in stdenv.mkDerivation rec {
       sed -i "s,^PATH=.*,PATH=$out/bin:${statdPath}," utils/statd/start-statd
 
       configureFlags="--with-start-statd=$out/bin/start-statd $configureFlags"
-      
+
       substituteInPlace systemd/nfs-utils.service \
         --replace "/bin/true" "${coreutils}/bin/true"