Move override of env var to make call of generate manifest clearer
Move override of env var to make call of generate manifest clearer

Co-Authored-By: Laurence de Bruxelles <28c80d0c8eb9f8be5f58dc3e782b116f71083396@digital.cabinet-office.gov.uk>
alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws