Adding INDEXED_SEARCH_INDEXER env var (#1208)

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Adding the `INDEXED_SEARCH_INDEXER` env var

### Checklist

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* [ ] Sister
[deploy-sourcegraph](https://github.com/sourcegraph/deploy-sourcegraph)
change:
* [ ] Sister
[customer-replica](https://github.com/sourcegraph/deploy-sourcegraph-docker-customer-replica-1)
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configuration):
* [ ] All images have a valid tag and SHA256 sum
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Author: marcleblanc2

docker-compose/docker-compose.yaml

@@ -136,6 +136,7 @@ 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'