fix: add missing on.workflow_call

See codeigniter4/shield#1056 (comment)

Author: kenjis

.github/workflows/label-add-conflict-all-pr.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
       - develop
+  workflow_call:
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true

.github/workflows/label-signing.yml

@@ -3,6 +3,7 @@ on:
   pull_request:
     branches:
       - 'develop'
+  workflow_call:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}