nix: reduce closure size of default package in flake.nix

Splitting the executable in a separate output avoids distributing all of
the libraries and documentation, which are not needed when just running
PostgREST.

Reduces closure size from 4.3G to 73.9M for the flake exported packages.

Resolves #4149

flake.nix

@@ -33,8 +33,8 @@
     in
     {
       packages = genSystems (attrs: {
-        default = attrs.postgrestPackage;
-        profiled = attrs.postgrestProfiled;
+        default = attrs.postgrestPackage.bin;
+        profiled = attrs.postgrestProfiled.bin;
       } // nixpkgs.lib.optionalAttrs (attrs ? postgrestStatic) {
         static = attrs.postgrestStatic;
       });
@@ -42,7 +42,7 @@
       apps = genSystems (attrs: {
         default = {
           type = "app";
-          program = "${attrs.postgrestStatic or attrs.postgrestPackage}/bin/postgrest";
+          program = "${attrs.postgrestStatic or attrs.postgrestPackage.bin}/bin/postgrest";
           meta.description = "REST API for any Postgres database";
         };
       });