Fix shutdown units

hosts/droppie/restic-backup.nix

@@ -37,7 +37,8 @@ in
   systemd.services."shutdown-after-backup" = {
     enable = true;
     serviceConfig = {
-      ExecStart = shutdownScript;
+      ExecStart = "${pkgs.bash}/bin/bash ${shutdownScript}";
+      Type = "oneshot";
     };
   };
 
@@ -47,5 +48,6 @@ in
       OnCalendar = "3..9:*";
     };
     wantedBy = [ "timers.target" ];
+    partOf = [ "shutdown-after-backup.service" ];
   };
 }