fix(i18n): Added missing textdomain (#1361)

## Problem

- The POT file update failed:
https://github.com/openSUSE/agama/actions/runs/9590615790/job/26446241862#step:22:8

## Solution

- Add the missing `textdomain` call

service/lib/agama/users.rb

@@ -37,6 +37,7 @@ class Users
     include Yast::I18n
 
     def initialize(logger)
+      textdomain "agama"
       @logger = logger
       update_issues
     end