tools/github actions: make build workflow reusable by other workflows

- https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_call
- https://github.com/awesome-selfhosted/awesome-selfhosted-data/actions/runs/5680604483
- error parsing called workflow: workflow is not reusable as it is missing a on.workflow_call trigger

.github/workflows/build.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches: [ master ]
   workflow_dispatch:
+  workflow_call:
 
 concurrency:
   group: build-${{ github.ref }}