Actually test changed code in e2e tests
Actually test changed code in e2e tests

Changed code in this repo is not actually tested in the
publishing-e2e-tests because the rename from Rummager to Search API was
not applied fully to publishing-e2e-tests which mean they expect a
RUMMAGER_COMMITISH when a SEARCH_API_COMMITISH is set. This explicitly
sets it.

alphagov/rummager,alphagov/rummager