Only retry failing QA tests in CI

What does this MR do?

Changes qa/spec/spec_helper.rb to only retry rspec tests automatically if they're run in CI.

This avoids having to go through retries when you're working on tests locally.

It's also consistent with /spec/spec_helper.rb

Depends on gitlab-qa!235 (merged) (which will be released as part of gitlab-qa!237 (merged) after gitlab-qa!236 (merged) is merged, which should be able to happen soon)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team