fix Ruby version

Author: bitbrain

.github/workflows/build-branch.yml

@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['3.0']
+        ruby-version: ['2.6']
     steps:
       - uses: ruby/setup-ruby@473e4d8fe5dd94ee328fdfca9f8c9c7afc9dae5e
         with: