ci(release): improve support for forks

[cpboyd]

github.repository will return uppercase characters if originally specified and GHCR only supports lowercase characters: https://github.com/orgs/community/discussions/27086

GitHub Actions doesn't natively provide any toLower function, so I've used Bash parameter expansion ${VAR_NAME,,} and re-exported the variable.

Additionally, the CHART_REPOSITORY path for the helm chart was hardcoded, so I've parameterized with github.repository like IMAGE_REPOSITORY to avoid a fork trying to push to oci://ghcr.io/kagent-dev/substrate/helm

• Tests pass
• Appropriate changes to documentation are included in the PR