use the API to trigger zfe builds
use the API to trigger zfe builds

Instead of pushing to trigger backend zfe builds, use the API to trigger
them. Using git to trigger zfe builds only works when the previous zfe
build and the current one are based on different SHAs, which won't be
the case if the production branch isn't updated between zfe builds.

By pushing a [ci skip] commit and triggering a zfe build using the API,
zfe builds will always get triggered.

circleci/frontend,circleci/frontend,circleci/frontend