make RUBYOPT available to all run steps

The earlier export was probably having no effect.

The circleci 'Set an environment variable' doc [1] states:

Since every run step is a new shell, environment variables are not
shared across steps. If you need an environment variable to be
accessible in more than one step, export the value using
BASH_ENV.

- [1]:
https://circleci.com/docs/set-environment-variable/#set-an-environment-variable-in-a-step

.circleci/config.yml

@@ -29,10 +29,11 @@ jobs:
       - run: gem --version
       - run: bundle --version
       - run: bundle install --gemfile=<< parameters.gemfile >>
-      - run: |
-          if [ "<< parameters.docker-image >>" > "ruby:2.2" ]; then
-            export RUBYOPT="--enable-frozen-string-literal"
-          fi
+      - unless:
+          condition:
+            equal: ["ruby:2.2", << parameters.docker-image >>]
+          steps:
+            - run: echo 'export RUBYOPT="--enable-frozen-string-literal"' >> "$BASH_ENV"
       - when:
           condition:
             equal: ["Gemfile", << parameters.gemfile >>]