Avoid race conditions when creating GpgSignature

What does this MR do?

This avoids errors when a race occurs when creating GPG signatures, it was surfaced by tests in https://gitlab.com/gitlab-org/gitlab-ce/issues/57304 but it also appears to happen in production https://gitlab.com/gitlab-org/gitlab-ce/issues/57304#note_138503204

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57304

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer