bumblebee: allow the use of different driver versions

[eadwu]

Motivation for this change

Not really perfect, though allows me to use the short-lived branch with a bit of extra configuration. nvidia_x11_beta is set to the short-lived branch on my nixpkgs.

  nixpkgs = {
    overlays = [
      (self: super: {
        bumblebee = super.bumblebee.override {
          nvidia_x11 = config.boot.kernelPackages.nvidia_x11_beta;
        };
      })
    ];
  };

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Fits CONTRIBUTING.md.

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[eadwu]

Using the solution from #49703 is better in my opinion.