Motivation for this change

Ignore the sanity check that prevents the Nvidia drivers from being built for kernels with real-time patches.
Even though the driver might not be officially supported by Nvidia for linux-rt, it seems to work without issues.
Attempting to install the driver for linux-rt fails with the following error message:

The kernel you are installing for is a PREEMPT_RT kernel!

The NVIDIA driver does not support real-time kernels. If you
are using a stock distribution kernel, please install
a variant of this kernel that does not have the PREEMPT_RT
patch set applied; if this is a custom kernel, please
install a standard Linux kernel.  Then try installing the
NVIDIA kernel module again.

*** Failed PREEMPT_RT sanity check. Bailing out! ***

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.