Squelch `find` executions in OpenBao Client gemspec

What does this MR do and why?

While running gitlab-ctl reconfigure or gitlab-rails console, I was seeing these messages spew:

$ sudo gitlab-rails c
find: ‘spec/*’: No such file or directory
find: ‘spec/*’: No such file or directory

Via strace I saw the execve() for find starting after opening openbao_client.gemspec.

Fix this by using the Ruby Dir path instead.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Install the latest Omnibus nightly.
2. Run gitlab-ctl reconfigure or gitlab-rails console and see this message.
3. Apply this patch to /opt/gitlab/embedded/service/gitlab-rails/gems/openbao_client/openbao_client.gemspec.
4. Repeat step 2 and see the messages are gone.