Pin GitHub Actions workflows to a specific version (#1603)

Author: ErikSchierboom

.github/workflows/exercise-tests.yml

@@ -19,7 +19,7 @@ jobs:
         ruby-version: [3.0, 3.1, 3.2]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@8575951200e472d5f2d95c625da0c7bec8217c42