fix(release): a snap built in a clone of its own is packed without --output, into the clone

snapcraft builds in LXD with the project mounted at /root/project and passes --output
on unchanged: naming the clone itself, it took the clone for a file in the folder above,
outside the project, and refused it again. Without --output it packs into the clone
(checked with snapcraft 9.2.1 in Todo's clone), from where the release tool moves it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

test/release/app_release_test.dart

@@ -167,18 +167,12 @@ void main() {
       );
     });
 
-    test('is packed inside its clone, as snapcraft packs nowhere outside the '
-        "project, and Learn's into the release's folder", () {
+    test('is packed without --output, into its clone, as snapcraft takes the '
+        "clone's own path for a file outside the project; Learn's into the "
+        "release's folder", () {
+      expect(snap.packOutput(root: '/repo', out: 'build/x'), isNull);
       expect(
-        snap.packPath(home: '/home/someone', root: '/repo', out: 'build/x'),
-        '/home/someone/.cache/halkyon-release/snap/halkyon-todo',
-      );
-      expect(
-        learnRelease.snap!.packPath(
-          home: '/home/someone',
-          root: '/repo',
-          out: 'build/x',
-        ),
+        learnRelease.snap!.packOutput(root: '/repo', out: 'build/x'),
         '/repo/build/x',
       );
     });