Set become_flags in ansible.cfg

Some users don’t have a login shell.

ansible.cfg

@@ -5,3 +5,4 @@ remote_user = root
 
 [privilege_escalation]
 become_method = su
+become_flags = -s /bin/sh -l

roles/forgejo/tasks/main.yml

@@ -142,7 +142,6 @@
     - name: Register runner
       become: yes
       become_user: forgejo-runner
-      become_flags: '-s /bin/sh -l'
       command: |
         forgejo-runner register --no-interactive --name runner
             --instance https://git.fri.uni-lj.si