Setup weekly cron on Github workflow (dart-lang/test_reflective_loader#29)

pkgs/test_reflective_loader/.github/workflows/build.yaml

diff --git a/pkgs/test_reflective_loader/.github/workflows/build.yaml b/pkgs/test_reflective_loader/.github/workflows/build.yaml
index 4d33943..2ac7525 100644
--- a/pkgs/test_reflective_loader/.github/workflows/build.yaml
+++ b/pkgs/test_reflective_loader/.github/workflows/build.yaml
@@ -5,6 +5,9 @@
   push:
     branches:
       - master
+  schedule:
+    # “At 00:00 (UTC) on Sunday.”
+    - cron: '0 0 * * 0'
 
 jobs:
   build: