Resolve "Flaky spec/services/packages/rubygems/create_gemspec_service_spec.rb:17"

What does this MR do?

Make sure we're looking at the right package_file

If we create a new package, the factory will also create a corresponding package file, which will share the same name as the package file we're creating explicitly, and it only has file_sha1 defined therefore if PostgreSQL happens to load the package file sharing the same name from the package, then it will fail.

This make sure we don't create a new package thus avoid having an extra package file which will conflict with the package file we're testing here.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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  • I have not included a changelog entry because _____.
• Documentation (if required)
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• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #326234 (closed)