linux/bootstrap: Use <nix/fetchurl> to get the static binaries

Since nix-1.4, nix's corepkgs contain a fetchurl suitable for
downloading the bootstrap binaries. Doing this will allow us to have a
nixpkgs with no in-tree binaries without breaking the purity of the
bootstrap (though for now, they are fetched in-tree until the binaries
are added to nixos.org somewhere). As an additional small benefit, the
in-tree binaries do not have to be hashed on every instantiation as they
do now.

The fetchurl in nix-1.2 and 1.3 isn't able to make binaries executable,
so it can't be used for this case. In that case, attempting to build the
bootstrap will show a message asking the user to manually download each
file and add it to the store with nix-store --add (but the hash is
ultimately the same, of course).

Signed-off-by: Shea Levy <shea@shealevy.com>