railgun integration test: "helm repo add" not idempotent

[mflendrich]

make -C ./railgun test.integration issues helm repo add kong which succeeds on the first run, but then fails on a subsequent run with an error that the kong repo already exists.

This does not surface on CI (because every run is in a clean environment) but gave me a spurious test error on my machine that took a while to debug.

[rainest]

There is a flag that should be able to work around this if it's not possible to ignore the error:

--force-update               replace (overwrite) the repo if it already exists

Not really its intended use, but 🤷