Mark ruby-head build as experimental

Author: mshibuya

.github/workflows/test.yml

@@ -29,7 +29,7 @@ jobs:
             experimental: false
           - ruby: ruby-head
             gemfile: gemfiles/rails-6-1.gemfile
-            experimental: false
+            experimental: true
           - ruby: jruby-head
             gemfile: gemfiles/rails-6-0.gemfile
             experimental: false