Set ChartDir only for directory-based charts to enable $ref resolution in
JSON schemas. Archived charts (.tgz) are loaded into memory without filesystem
extraction, so $ref resolution is not supported for them.
This fixes the original issue where `helm template .` and `helm install .`
failed to validate schemas with relative $ref references.
Fixes#31260
Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>