nixos/kernel.nix: ensure same kernel is used
nixos/kernel.nix: ensure same kernel is used

Originally, changes to the kernel don't propagate to the other
derivation within the same package set. This commit allows for the
changes in the kernel to be propagated.

A distinct example is setting `boot.kernel.randstructSeed` to a non-zero
length string which would result in building 2 kernels, one with the
correct seed and the other with the zero length seed. Then, when using
an out-of-tree kernel driver, it would be built with the zero length
seed which differs from the non-zero length seed used to boot,
contradicting the purpose of the `boot.kernel.randstructSeed`.

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