drone: fix git fetch for main, too

.drone.yml

@@ -36,10 +36,10 @@ steps:
       GITEA_SSH_KEY:
         from_secret: gitea_ssh_key
     commands:
+      - git checkout origin/main
       - ./.drone/setup_ssh.sh
       - git remote set-url origin git@git.b12f.io:pub-solar/os
       - git fetch --all
-      - git checkout origin/main
       - ./.drone/upstream-branch.sh
 
   - name: "Open pull request for failed merge"