fix: only generate a version file if nvidiaVersion isn't set

[cfhammill]

When using nixGL in a flake, it would be convenient to use the auto.nixGLDefault attribute to generate a nixGL wrapper (previously I was generating nixGLnvidia-<version>.

auto.nixGLDefault can't be used in a flake because there are two impure operations, first is generating the version file, and second is fetching the sha256 for the runfile.

This PR prevents those two operations from occuring if the user provides nvidiaVersion and nvidiaHash arguments to nixGL

Hunk 1: prevents the version file generation if nvidiaVersion is passed, see the comment in the code for why this works
Hunk 2: simply passes nvidiaHash to nvidiaPackages, in the case nvidiaHash is null, this should fetch the hash as before.