Fix CI setting for Ruby 3.0 (#326)

Co-authored-by: Piotr Kuczynski <[email protected]>

Author: ytkg

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
         ruby:
           - 2.6
           - 2.7
-          - 3.0
+          - '3.0' # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
           - 3.1
           - jruby
           - truffleruby-21