Drop the assertion around the netrc under dnixd

[grahamc]

As it stands, determinate + magic-nix-cache seems universally disabled, because the netrc path is being specified. After #102 not sure where this leaves it.

Not sure the implications of this, but let's try this.

[colemickens]

Oh I'm so sorry to do this to you, but also, it's good timing, because now the right choice is more obvious.

So, the issue is that we /still use/ the user specified netrc path to read the FH password. And if they tell us the wrong path, we won't be able to load the right password and init the flakehub cache anyway.

So after just telling you this is fine, I think it's not fine, and I think the best solution is to make magic-nix-cache read your new auth token you just merged, and remove this netrc parsing entirely. Or something step-wisey to get approximately there?