Remove unnecessary Storybook yarn installs in CI

What does this MR do and why?

This removes the storybook yarn install from the default yarn_install_script command and separates it into its own yarn_install_script_storybook, which we now only call selectively in the storybook-relevant jobs. This should speed up the frontend CI critical path¹ by up to 60 seconds (15-25 seconds per install, across three jobs). We appear to not need to install storybook in most of these jobs but I’m not sure.

The alternative would be to also pull the storybook cache anytime we run yarn_install_script but this seems unnecessary (?) and would also introduce a (admittedly smaller) latency.

¹: appears to be compile-test-assets -> rspec-all frontend_fixture (doesn’t install yarn deps) -> jest -> coverage-frontend?