Use flake.nix for the project root when dealing with flakes (#364)

The default value for `config.projectRootFile` is `.git/config`, which
is a nice default that doesn't assume flakes (because flakes are more
controversial than git in the nix community :))

However, if you're using flake-parts, then you're definitely OK with
flakes, and using `flake.nix` as a project root file makes sense.

As a consequence of this, subflakes now probably do what you expect. In
other words, this fixes #357.

Side note: in the future, nix will expose a `PRJ_ROOT` env var that we could use
instead. See
NixOS/nix@7635874.

Author: jfly

flake-module.nix

@@ -62,6 +62,8 @@ in
                     Path to the root of the project on which treefmt operates
                   '';
                 };
+
+                config.projectRootFile = lib.mkDefault "flake.nix";
               }
             ];
           };