caddy-frontend: Fix backend Via header for https backend

Rarely used https backend was missing the backend Via additions, thus it was
not transmitted there.
4 jobs for feature/caddy-frontend-cleanup-202207 in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

02:52:53

failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

02:52:33

failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

03:00:16

failed SlapOS.SoftwareReleases.IntegrationTest-py3-luke/feature/caddy-frontend-cleanup-202207

02:55:44