MAINT: update linkchecker to run weekly (#379)

Author: mmcky

.github/workflows/linkcheck.yml

@@ -2,7 +2,7 @@ name: Link Checker [Anaconda, Linux]
 on:
   schedule:
     # UTC 23:00 is early morning in Australia (9am)
-    - cron:  '0 23 * * *'
+    - cron:  '0 23 * * 1'
   workflow_dispatch:
 jobs:
   link-checking: