Set default flake directory as /etc/nix-darwin

Backported from https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh#L349-L353

pkgs/nix-tools/darwin-rebuild.sh

@@ -26,6 +26,7 @@ extraProfileFlags=()
 profile=@profile@
 action=
 flake=
+noFlake=
 
 while [ $# -gt 0 ]; do
   i=$1; shift 1
@@ -69,6 +70,9 @@ while [ $# -gt 0 ]; do
       flake=$1
       shift 1
       ;;
+    --no-flake)
+      noFlake=1
+      ;;
     -L|-vL|--print-build-logs|--impure|--recreate-lock-file|--no-update-lock-file|--no-write-lock-file|--no-registries|--commit-lock-file|--refresh)
       extraLockFlags+=("$i")
       ;;
@@ -120,6 +124,14 @@ if [ -z "$action" ]; then showSyntax; fi
 
 flakeFlags=(--extra-experimental-features 'nix-command flakes')
 
+# Use /etc/nixos/flake.nix if it exists. It can be a symlink to the
+# actual flake.
+if [[ -z $flake && -e /etc/nix-darwin/flake.nix && -z $noFlake ]]; then
+    flake="$(dirname "$(readlink -f /etc/nix-darwin/flake.nix)")"
+fi
+
+# For convenience, use the hostname as the default configuration to
+# build from the flake.
 if [ -n "$flake" ]; then
     # Offical regex from https://www.rfc-editor.org/rfc/rfc3986#appendix-B
     if [[ "${flake}" =~ ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))? ]]; then