set build triggers

- trigger builds on Tuesday and Saturday
  the build times exceed 24 hours and are expected to go up, so more
  than two a week does not make too much sense.
- do not trigger on changes to user scripts and documentation

.github/workflows/build.yaml

@@ -3,6 +3,11 @@ name: build
 on:
   workflow_dispatch:
   push:
+    paths:
+      - '**.repos'
+      - '**.yaml'
+  schedule:
+    - cron: '13 1 * * 2,6'
 
 jobs:
   ros-robot: