build: release 6.50.0 hotfix

[LinHuiqing]

Problem

• In release 6.50.0, we introduced a bug (f4f5998) where payment-related copy meant for non-production environments shown on production environments.
• This PR fixes it by making the SECRET_ENV environment variable accessible on frontend through useEnv.

Tests

• On a storage-mode form that does not have payments enabled, go to payment settings page and check that the copy is correct and links to the guide.
• When the SECRET_ENV is set as production:
  • Go to a storage-mode form with payment enabled and preview the form. There should be a banner and check that the copy is expected as above.
  • Go to a storage-mode form with payment enabled, open the form and try to pay as a respondent. On the payment page, check that there is no infobox above the payment element.
• When the SECRET_ENV is not set as production (e.g. staging):
  • Go to a storage-mode form with payment enabled and preview the form. There should be a banner and check that the copy is expected as above.
  • Go to a storage-mode form with payment enabled, open the form and try to pay as a respondent. On the payment page, check that there is an infobox with the expected copy on the test card.

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #6350. Mergify cannot evaluate rules on this PR. ⚠️