Use concurrency for GitHub Actions workflow

Same as: rubocop/rubocop-rspec#1539

This PR ensures that only the last execution per branch always works, even when pushed multiple times in a row, for example. The older one is canceled.

Refs:
https://docs.github.com/en/actions/using-jobs/using-concurrency https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/

.github/workflows/rubocop.yml

@@ -9,6 +9,10 @@ on:
       - master
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   ast_specs:
     name: >-

.github/workflows/spell_checking.yml

@@ -2,6 +2,10 @@ name: Spell Checking
 
 on: [pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   codespell:
     name: Check spelling of all files with codespell