Add path protection on push for workflows/boards_build.

According to the rules at https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#patterns-to-match-file-paths

.github/workflows/boards_build.yml

@@ -2,6 +2,9 @@
 name: Build supported boards
 on:
   push:
+    paths:
+     - 'patches/tock/*'
+     - 'third_party/tock/**'
   pull_request:
     types: [opened, synchronize, reopened]