Apply concurrency for GHA workflow pull_requests envents only

rubocop/rubocop#11414

Author: koic

.github/workflows/linting.yml

@@ -2,7 +2,7 @@ name: Linting
 on:
   - pull_request
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 jobs:
   yamllint:

.github/workflows/spell_checking.yml

@@ -3,7 +3,7 @@ name: Spell Checking
 on: [pull_request]
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs: