add ruby 2.6 key to yml

this workflow didn't run, probably because it's a syntax error to have
two gemfile entries for a single ruby key (I wondered if I could have a
single gemfile array keyval, but I'm trying to do the obvious thing first)

.github/workflows/tests.yml

@@ -23,6 +23,7 @@ jobs:
             gemfile: gemfiles/rails_5_2.gemfile
           - ruby: '2.6'
             gemfile: gemfiles/rails_7_0.gemfile
+          - ruby: '2.6'
             gemfile: gemfiles/rails_7_1.gemfile
 
     steps: