Set default flake directory as /etc/nix-darwin

Backported from https://github.com/NixOS/nixpkgs/blob/9995a5d59c59fde96f4ad7d811a8ed832357440f/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh#L349-L353

pkgs/nix-tools/darwin-rebuild.sh

@@ -34,6 +34,7 @@ extraProfileFlags=()
 profile=@profile@
 action=
 flake=
+noFlake=
 
 while [ $# -gt 0 ]; do
  i=$1; shift 1
@@ -77,6 +78,9 @@ 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")
  ;;
@@ -137,6 +141,19 @@ if [ -z "$action" ]; then showSyntax; fi
 
 flakeFlags=(--extra-experimental-features 'nix-command flakes')
 
+# Search the default locations for the system-wide nix-darwin configuration
+if [[ -z $flake && -z $noFlake ]]; then
+ if [[ -e /etc/nix-darwin/flake.nix ]]; then
+ flake="$(dirname "$(readlink -f /etc/nix-darwin/flake.nix)")"
+ elif [[ -e ~/.config/nix-darwin/flake.nix ]]; then
+ flake="$(dirname "$(readlink -f ~/.config/nix-darwin/flake.nix)")"
+ elif [[ -e ~/.nixpkgs/flake.nix ]]; then
+ flake="$(dirname "$(readlink -f ~/.nixpkgs/flake.nix)")"
+ fi
+fi
+
+# For convenience, use the hostname as the default configuration to
+# build from the flake.
 if [ -n "$flake" ]; then
  # Offical regex from https://www.rfc-editor.org/rfc/rfc3986#appendix-B
  if [[ "${flake}" =~ ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))? ]]; then