Add INDEXED_SEARCH_INDEXER to sourcegraph-frontend-internal service envs (#1209)

Add `INDEXED_SEARCH_INDEXER` to sourcegraph-frontend-internal service
ENVs. Follow up to
#1208

### Checklist

<!--
Kubernetes and Docker Compose MUST be kept in sync. You should not merge
a change here
without a corresponding change in the other repository, unless it truly
is specific to
this repository. If uneeded, add link or explanation of why it is not
needed here.
-->
* [ ] Sister
[deploy-sourcegraph](https://github.com/sourcegraph/deploy-sourcegraph)
change:
* [ ] Sister
[customer-replica](https://github.com/sourcegraph/deploy-sourcegraph-docker-customer-replica-1)
change (if necessary, for any changes affecting pure-docker or
configuration):
* [ ] All images have a valid tag and SHA256 sum
### Test plan

CI
<!--
As part of SOC2/GN-104 and SOC2/GN-105 requirements, all pull requests
are REQUIRED to
provide a "test plan". A test plan is a loose explanation of what you
have done or
implemented to test this, as outlined in our Testing principles and
guidelines:

https://docs.sourcegraph.com/dev/background-information/testing_principles
  Write your test plan here after the "Test plan" header.
-->

Author: loujar

docker-compose/docker-compose.yaml

@@ -188,6 +188,7 @@ services:
       - 'SRC_GIT_SERVERS=gitserver-0:3178'
       - 'SRC_SYNTECT_SERVER=http://syntect-server:9238'
       - 'SEARCHER_URL=http://searcher-0:3181'
+      - 'INDEXED_SEARCH_INDEXER=zoekt-indexserver-0:6072'
       - 'INDEXED_SEARCH_SERVERS=zoekt-webserver-0:6070'
       - 'SRC_FRONTEND_INTERNAL=sourcegraph-frontend-internal:3090'
       - 'GRAFANA_SERVER_URL=http://grafana:3370'