fix: buildFromSdist default for haskell-flake managed packages only

Author: srid

nix/modules/project/defaults.nix

@@ -144,6 +144,7 @@ in
         # Disabling haddock and profiling is mainly to speed up Nix builds.
         haddock = lib.mkDefault false; # Because, this is end-user software. No need for library docs.
         libraryProfiling = lib.mkDefault false; # Avoid double-compilation.
+        buildFromSdist = lib.mkDefault true; # Ensure release-worthiness
       };
     };
 
@@ -193,9 +194,7 @@ in
       defaultText = ''
         Make sure all files we use are included in the sdist, as a check for release-worthiness.
       '';
-      default = {
-        buildFromSdist = lib.mkDefault true;
-      };
+      default = { };
     };
 
     projectModules.output = mkOption {