Fix CI setting for Ruby 3.0

Author: ytkg

.github/workflows/tests.yml

@@ -5,7 +5,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 3.0, 3.1, head]
+        # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
+        ruby: [2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, '3.0', 3.1, head]
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2