[Backport 6.11.x] fix(ci): helm unittest is broken (#788)

see comments 

### Test plan

CI pass now
 <br> Backport b9369b8 from #787

Co-authored-by: Michael Lin <[email protected]>

Author: sourcegraph-release-bot

scripts/ci/helm-unittest.sh

@@ -2,8 +2,12 @@
 
 set -euf -o pipefail
 
+# 1.0.3 is broken now
+# https://github.com/helm-unittest/helm-unittest/issues/790
+HELM_UNITTEST_VERSION="v1.0.2"
+
 ### Install the helm-unittest plugin
-helm plugin install https://github.com/helm-unittest/helm-unittest
+helm plugin install https://github.com/helm-unittest/helm-unittest --version "$HELM_UNITTEST_VERSION"
 
 ### Run the helm tests
 helm unittest -q charts/sourcegraph