fix: add a cron check to catch upstream breaks

This library will probably go "cold" soon since it is feature complete,
so the CI pipe won't be triggered often. But would like to be alerted
to breaks.

Author: nyonson

.github/workflows/weekly-check.yml

@@ -0,0 +1,18 @@
+# Attempt to detect any upcoming breaking changes if CI hasn't been run in awhile.
+name: Weekly Check
+
+on:
+  # Allows manual triggering.
+  workflow_dispatch:  
+  schedule:
+   # Run at midnight on Sundays.
+   - cron: "0 0 * * 0" 
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: extractions/setup-just@v3
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+      - run: just check