[e.cash] Fix preview builds

Summary: Disable https CSP header during e.cash preview builds

Test Plan:
./contrib/teamcity/build-configurations.py preview-e.cash

note the url from the Preview is available at http://127.0.0.1:41868 for the next 60 minutes. output

curl -I http://127.0.0.1:41868 and no headers

Then, build a prod-like docker instance:

docker build -t ecash_local .
docker run -it --rm -p 3000:3000 ecash_local
curl -I http://localhost:3000

note the expected headers are there

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15268