linux: disable module signing
linux: disable module signing

Without this, the kernel would generate a random one for us which obviously
isn't reproducible.

`nix-build -A linux --check` succeeds now!
(Tested at different times with different kernel)

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs