flake.nix: add aarch64-darwin

Changed files
+1
+1
flake.nix
···
"aarch64-linux"
"armv6l-linux"
"armv7l-linux"
];
forAllSystems = f: lib.genAttrs systems (system: f system);
···
"aarch64-linux"
"armv6l-linux"
"armv7l-linux"
+
"aarch64-darwin"
];
forAllSystems = f: lib.genAttrs systems (system: f system);