[Backport 7.4.x] bug/release: fix test to work during internal release build (#890)

Backport 8db7e22 from #889

Fix tests introduced in
#885 to work
during the release flow.

This test broke the release for the internal build
#888

### Checklist

- [ ] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [ ] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [ ] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

Co-authored-by: Warren Gifford <warrenbruceg@gmail.com>

Author: sourcegraph-release-bot

charts/sourcegraph/tests/imageRepository_test.yaml

@@ -6,6 +6,9 @@ tests:
 - it: should use the global repository by default
   template: frontend/sourcegraph-frontend.Deployment.yaml
   set:
+    sourcegraph:
+      image:
+        repository: index.docker.io/sourcegraph
     frontend:
       image:
         defaultTag: test
@@ -54,6 +57,9 @@ tests:
 - it: should only affect the targeted service and leave others on the global repository
   template: frontend/sourcegraph-frontend.Deployment.yaml
   set:
+    sourcegraph:
+      image:
+        repository: index.docker.io/sourcegraph
     frontend:
       image:
         defaultTag: test