Update flake.nix to allow proper cross-compiling

[SomeoneWeird]

This lets us properly cross compile from x86_64-linux to aarch64-linux, hopefully to allow the whole OS to be cross compiled.

[nix-shell:~/dkm]$ uname -a
Linux collie 6.6.69 #1-NixOS SMP PREEMPT_DYNAMIC Thu Jan  2 09:32:11 UTC 2025 x86_64 GNU/Linux

[nix-shell:~/dkm]$ nix build .#cross.aarch64-linux --print-out-paths
warning: Git tree '/home/adam/dkm' is dirty
/nix/store/fsnijwilrkh08zpr98wrf4hvgip05v94-dkm-unstable

[nix-shell:~/dkm]$ file /nix/store/fsnijwilrkh08zpr98wrf4hvgip05v94-dkm-unstable/bin/dkm
/nix/store/fsnijwilrkh08zpr98wrf4hvgip05v94-dkm-unstable/bin/dkm: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /nix/store/cv1868kzij6dl9101xlpxwklisc407gk-glibc-aarch64-unknown-linux-gnu-2.40-66/lib/ld-linux-aarch64.so.1, for GNU/Linux 3.10.0, Go BuildID=4mmC7EHgazFtjqhyN8Dn/_lHbM7X9YS5AtzutJgjC/trgtFwMuh2fDw64LRsLm/RvuwVN7aQ4w7_YYjFygb, with debug_info, not stripped