switch to predefined license for kmod-*-nvidia

[bcressey]

The NVIDIA kmod packages are currently the only consumer of the "bring-your-own-license" (BYOL) support in the build system.

This feature has caused various headaches since it came into use, such as #2294. However the real pain is with OOTBs as tracked in bottlerocket-os/twoliter#100. Any package that needs to be distributed in a kit can't reasonably extend the BYOL property to downstream trees, because the package will already have been built into an RPM with license metadata and a copy of the software license selected by the distributor.

So it's time to withdraw support and use a predefined license for these packages. That will be LicenseRef-NVIDIA-AWS-EULA which is the license that official Bottlerocket images have always specified within the final images.