Fix hvc codes in Export win dataset.

[elcct]

Description of change

The new HVC codes don't have legacy win id, so they were not showing up correctly in the Export win dataset.
This reconstructs the id from campaign_id and financial_year.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (027a996) to head (fbbf6e7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5404   +/-   ##
=======================================
  Coverage   96.39%   96.39%           
=======================================
  Files         938      938           
  Lines       22326    22326           
  Branches     2002     2002           
=======================================
  Hits        21522    21522           
  Misses        655      655           
  Partials      149      149           

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