maint: Set formatter Nix flake output so nix fmt works

I've idly thought about making the formatter a derivation that runs alejandra
over Nix code, then rustfmt over Rust code, etc--so that just running `nix
fmt` in the root cleans up everything--but I haven't made that work yet.

flake.nix

@@ -45,6 +45,7 @@
       };
     in {
       packages.default = heracles;
+      formatter = pkgs.alejandra;
     })
     // {
       nixosModule = {