Use repository_dispatch API instead of workflow_dispatch for build trigger

The PAT lacks actions:write permission needed for `gh workflow run`.
Switch to the repository_dispatch API which only requires
contents:write, a permission the token already has for git push.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/workflows/update.yml

@@ -28,6 +28,6 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.MATZBOT_AUTO_UPDATE_TOKEN }}
       - name: dispatch build
         if: steps.commit.outputs.updated == 'true'
-        run: gh workflow run build.yml
+        run: gh api repos/${{ github.repository }}/dispatches -f event_type=build
         env:
           GH_TOKEN: ${{ secrets.MATZBOT_AUTO_UPDATE_TOKEN }}