Fix format script

The previous script was failing, as it was supposed to.
It was trying to match a file named `'**/*.nix'`, rather
than expanding the intended pattern.

Bash doesn't do shell expansion inside of quotes.
It must be written unquoted, else `*` and other
special characters are treated literally.

`'**/*.nix'` -> `**/*.nix`

The new version will expand correctly.

Additionally, I've enabled `globstar` to make the
expansion match all files ending with `.nix`
recursively, including the top level. Without this,
the pattern will only match up one level.

`globstar` is documented here:
<https://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html>

flake.nix

@@ -27,7 +27,11 @@
           format = pkgs.writeShellApplication {
             name = "format";
             runtimeInputs = with pkgs; [ nixpkgs-fmt ];
-            text = "nixpkgs-fmt '**/*.nix'";
+            text = ''
+              shopt -s globstar
+
+              nixpkgs-fmt -- **/*.nix
+            '';
           };
 
           # only run this locally, as Actions will run out of disk space