Format whole flake when fmt has no args

Now that Nix/Lix do not pass `.` by default for `nix fmt` we can take
`nix fmt` without an arg to mean to format the entire flake.

builtinModules/formatter.nix

@@ -42,7 +42,17 @@ in
           in
           pkgs.writeShellScriptBin "formatter" ''
             PATH=${if fullContext then "" else makeBinPath packages}
-            if [ $# -eq 0 ]; then exec "$0" .; fi
+            if [ $# -eq 0 ]; then
+              flakedir=.
+              while [ "$(${coreutils}/bin/realpath "$flakedir")" != / ]; do
+                if [ -e "$flakedir/flake.nix" ]; then
+                  exec "$0" "$flakedir"
+                fi
+                flakedir="$flakedir/.."
+              done
+              echo Failed to find flake root! >&2
+              exit 1
+            fi
             for f in "$@"; do
               if [ -d "$f" ]; then
                 ${fd}/bin/fd "$f" -Htf -x "$0" &