Pin GitHub Actions workflows to a specific version (#103)

Author: ErikSchierboom

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5
 
       - name: Run tests for all exercises
         run: bin/test