fix(ci): Use correct `ENV` value for production

[khvn26]

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Changes

This provides correct ENV value to E2E tests ran against production.

How did you test this code?

This is a CI change.

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3 Ignored Deployments

| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **docs** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/docs/9U28uX981bNGu2FKB36akm2tPoMu)) | | | Jun 25, 2024 8:32pm | | **flagsmith-frontend-preview** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-preview/3tSPuh5oVx5D1ddCXBXUdwzBkW8D)) | | | Jun 25, 2024 8:32pm | | **flagsmith-frontend-staging** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-staging/AMWU6f6kxegYQPWezmdYFAcXYU5u)) | | | Jun 25, 2024 8:32pm |

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.57%. Comparing base (a3bae58) to head (8c52f51).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4237 +/- ## ======================================= Coverage 96.57% 96.57% ======================================= Files 1190 1190 Lines 38754 38754 ======================================= Hits 37426 37426 Misses 1328 1328 ```

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