fix: forward nvidiaHash for nixgl.auto derivation too

[muggenhor]

This allows separating the use of nixgl.auto.nixGLDefault from the location where we pass nvidiaHash to provide a pure instantiation.

Without this usage of nixgl.auto.XXX is always impure.

Also reuse nixgl.auto in top when nVidia is forced instead of duplicating its definition, because that leaves the possibility for bugs when these duplicates aren't in sync.