helm-op: repositories.yaml is sensitive to (missing) trailing slash

[squaremo]

In #1726, a large part of the problem was that missing the trailing slash from the repository URL in the HelmRelease causes a couple of problems:

1. It can mean helm-op doesn't find authentication details in credentials.yaml, if they happen to have the trailing slash
2. When downloading a chart tarball, the constructed URL will be missing a path element if there's no trailing slash (e.g., https://myrepo/helm/charts + mychart-1.0.1.tar = https://myrepo/helm/mychart-1.0.1.tar).