The plugin runs under both Vault and OpenBao, but nfpm only produced a
single RPM that installs into /opt/vault-plugins. OpenBao hosts expect
their plugins under /opt/openbao-plugins, so a second package is needed.
- Parameterize nfpm.yaml with PACKAGE_NAME, PACKAGE_PLUGIN_DIR, and
PACKAGE_PREINSTALL so one config renders per target server
- Replace the static preinstall.sh with preinstall.sh.tmpl that
mkdir -p's the flavour's plugin directory
- Build two RPMs in build-rpm.sh via a build_flavor helper:
vault-plugin-secrets-litellm -> /opt/vault-plugins and
openbao-plugin-secrets-litellm -> /opt/openbao-plugins