flake.nixpkgs: use nix_2_18 from unstable #602

Job	Run time
nix (aarch64, fmt -- --check)	3m 51s
nix (aarch64, eval .#apps.$_system.nixos-shell.program)	3m 50s
nix (aarch64, eval .#apps.$_system.setup.program)	3m 51s
nix (x86_64, fmt -- --check)	1s
nix (x86_64, eval .#apps.$_system.nixos-shell.program)	1s
nix (x86_64, eval .#apps.$_system.setup.program)	1s
nix (develop .#jdk8 --profile profile, x86_64)	3m 54s
nix (develop .#jdk11 --profile profile, x86_64)	3m 50s
nix (develop .#jdk17 --profile profile, x86_64)	3m 49s
nix (develop .#php74 --profile profile, x86_64)	1m 2s
nix (develop .#php74-composer1 --profile profile, x86_64)	1m 15s
nix (develop .#php80 --profile profile, x86_64)	49s
nix (develop .#php81 --profile profile, x86_64)	1s
nix (build .#homeConfigurations."tobias@gamer".activationPackage, x86_64)	2m 42s
nix (build .#nixOnDroidConfigurations.pixel7a.activationPackage --impure, aarch64)	3m 50s
nix (build .#nixosConfigurations.argon.config.system.build.toplevel, aarch64, argon)	3m 52s
nix (build .#nixosConfigurations.krypton.config.system.build.toplevel, x86_64, krypton)	3m 5s
nix (build .#nixosConfigurations.neon.config.system.build.toplevel, x86_64, neon, --async)	3m 41s
nix (build .#nixosConfigurations.xenon.config.system.build.toplevel, aarch64, xenon)	3m 49s
nix (build .#packages.$_system.rpi-firmware, aarch64)	3m 50s
nix (build .#packages.$_system.rpi-image, aarch64)	3m 50s
nix (build .#packages.$_system.installer-image, x86_64)	2m 51s
	57m 45s

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