workflow: Simplify automatic merge conflict solving

.gitea/workflows/update-flake.yaml

@@ -34,12 +34,7 @@ jobs:
             if [ $commits_ahead -ne 0 ]
             then
               git fetch origin
-              git merge --squash origin/main || true # do not exit script on merge conflict
-
-              # solve potential merge conflict by taking flake.lock from main
-              git checkout --theirs flake.lock
-              git add flake.lock
-
+              git merge -X theirs --squash origin/main
               git commit -m "Merge main branch squashed $(date -I)"
             else
               echo "Merge not necessary"