ci: avoid passing enable-frozen-string-literal flag to ruby

This flag was redundant for our own code since we already have the
`# frozen_string_literal: true` directive at the top of each file,
enforced by RuboCop.

While the flag could theoretically catch frozen string issues in
dependencies, I don't think we want our CI to fail because of frozen
string errors in gems we don't have control of.

Author: santiagorodriguez96

.github/workflows/build.yml

@@ -70,8 +70,6 @@ jobs:
         bundler-cache: true
 
     - run: bundle exec rspec
-      env:
-        RUBYOPT: ${{ startsWith(matrix.ruby, '3.4') && '--enable=frozen-string-literal' || '' }}
 
   lint:
     runs-on: ubuntu-latest