fix: Reference schema files from the installed package for helm lint

[okurz]

Motivation:

make test-helm-lint failed locally on openSUSE Slowroll as of 2026-03
because ct (Chart Testing tool) cannot find the default schema files
chart_schema.yaml and lintconf.yaml. This is likely due to a newer
chart-testing package (at time of writing problem appears with
3.14.0-1.2), which places these files in a non-standard location
(/usr/share/doc/packages/chart-testing/) instead of the default search
paths (., ~/.ct/, or /etc/ct/).

Design Choices:

• Updated tools/test_helm_chart to check for schema files in the
  default search locations first.
• If not found, it attempts to locate them in the package's
  documentation directory and passes them explicitly via
  --chart-yaml-schema and --lint-conf.
• This approach avoids committing large schema files to the repository
  while ensuring compatibility with the newer system's package layout.

Benefits:

• Restores the functionality of make test-helm-lint for local test
  runs and/or on newer systems
• Ensures robustness across different installation layouts of the
  chart-testing tool.