CI: pin GitHub Actions workflows (#767)

* Pin GitHub Actions workflows to a specific version

* Apply suggestions from code review

Author: ErikSchierboom

.github/workflows/config-checker.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
 
       - name: Check Config
         uses: ./.github/actions/perform-system

.github/workflows/configlet-sync.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-22.04
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
 
     - name: Fetch configlet
       uses: exercism/github-actions/configlet-ci@main

.github/workflows/test-exercises.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
 
       - name: Run Tests
         uses: ./.github/actions/perform-system