nix: fix allowUnfreePredicate for packages with multiple licenses (#19237)

nix: fix allowUnfreePredicate for packages with multiple licenses (#19237)

The allowUnfreePredicate in pkgsCuda was wrapping p.meta.license in a
list unconditionally. This fails when meta.license is already a list
of licenses, as it creates a nested list and then tries to access
.free and .shortName on the inner list.

Use lib.toList instead, which correctly handles both cases:
- Single license attrset -> wraps in list
- List of licenses -> returns unchanged