workflows/eval: fix eval report with formatting changes

The Eval report which tests performance between Nix/Lix versions on
update of `ci/pinned` wrongly returned errors, when only the special
attribute `release-checks` changed. Since this reads in all of Nixpkgs,
it will change with any formatting change that is introduced at the same
time via update of any of `treefmt`'s formatters.

.github/workflows/eval.yml

@@ -279,12 +279,11 @@ jobs:
                         const diff = JSON.parse(
                           readFileSync(path.join(artifact, system, 'diff.json'), 'utf-8'),
                         )
-                        const attrs = [].concat(
-                          diff.added,
-                          diff.removed,
-                          diff.changed,
-                          diff.rebuilds
-                        )
+                        const attrs = []
+                          .concat(diff.added, diff.removed, diff.changed, diff.rebuilds)
+                          // There are some special attributes, which are ignored for rebuilds.
+                          // These only have a single path component, because they lack the `.<system>` suffix.
+                          .filter((attr) => attr.split('.').length > 1)
                         if (attrs.length > 0) {
                           core.setFailed(
                             `${version} on ${system} has changed outpaths!\nNote: Please make sure to update ci/pinned.json separately from changes to other packages.`,