Fix(Productivity Analytics): store initialization

What does this MR do?

This MR fixes an issue introduced here, where the exported store was changed to a function instead of an initialized store. This MR corrects that mistake.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

To test this fix navigate to this page and apply filters, you should see the results now being filtered by the applied filters:

• http://localhost:3000/groups/flightjs/-/analytics/productivity_analytics

• 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

Closes #255887 (closed)