Run all tests with frozen string literals enabled

To catch any accidental errors mutating frozen strings, enable them by
default in CI.

See https://ruby.social/@byroot/112177840865649404

.github/workflows/tests.yml

@@ -1,14 +1,19 @@
 name: re2 Tests
+
 concurrency:
   group: "${{github.workflow}}-${{github.sha}}"
   cancel-in-progress: true
+
 on:
   workflow_dispatch:
   push:
     branches:
       - main
   pull_request:
 
+env:
+  RUBYOPT: "--enable-frozen-string-literal --debug-frozen-string-literal --debug"
+
 jobs:
   build-cruby-gem:
     name: "Build CRuby gem"