GitHub Action: weekly cron (#147)

diff --git a/.github/workflows/build.yaml b/.github/workflows/build.yaml
index 446e513..20aa3a6 100644
--- a/.github/workflows/build.yaml
+++ b/.github/workflows/build.yaml
@@ -1,6 +1,9 @@
 name: Dart
 
 on:
+  schedule:
+    # “At 00:00 (UTC) on Sunday.”
+    - cron: '0 0 * * 0'
   pull_request:
   push:
     branches: