Fix inconsistent Tailwind CSS generation

What does this MR do and why?

In gitlab-com/gl-infra/production#18107 (closed)#, we saw that the compile-production-assets CI job in GitLab Rails produced slightly different Tailwind CSS files from the files generated by Cloud Native GitLab gitlab-rails-ee image.

This appears to have caused by a subtle difference in how the two jobs ran. The compile-production-assets job attempts to load a cached version by calculating a hash from the source files via bundle exec rake gitlab:assets:hash_sum. However, the gitlab-rails-ee job doesn't do that.

The hash_sum Rake task has a subtle side effect of invoking the gitlab:assets:tailwind Rake task, which runs the tailwind_all_the_way.mjs script. This causes an inconsistent results, depending on how many times the gitlab:assets:tailwind was invoked because:

1. tailwind_all_the_way.mjs depends on tailwind.config.js, which attempts to load config/helpers/tailwind/css_in_js.js if it is present.
2. However, tailwind_all_the_way.mjs generates config/helpers/tailwind/css_in_js.js, so the tailwind.config.js could load different rules after config/helpers/tailwind/css_in_js.js is built.
3. Ultimately the generated tailwind.css may be different depending on whether config/helpers/tailwind/css_in_js.js existed in the first place.

To ensure that tailwind.css is in the correct state, run the gitlab:assets:tailwind Rake task twice in a row during the gitlab:assets:compile Rake task. This ensures that the assets will always be consistent if the CI job doesn't run bundle exec rake gitlab:assets:hash_sum.

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How to set up and validate locally

In master, run:

1. rm -rf config/helpers/tailwind
2. bundle exec rake gitlab:assets:compile
3. Then run bundle exec rake gitlab:assets:hash_sum. That is hash A.
4. Repeat step 2 and verify the hash is different this time.

With this branch, the hash should be the same.