Pin GitHub Actions runners to a specific version (#676)

Author: ErikSchierboom

.github/workflows/exercises.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: ['ubuntu-latest', 'macos-latest', 'windows-latest']
+        os: ['ubuntu-22.04', 'macos-12', 'windows-2022']
     name: Rakudo Star on ${{ matrix.os }}
     steps:
       - name: Checkout

.github/workflows/generator.yml

@@ -5,7 +5,7 @@ on:
 
 jobs:
   generator:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout
         uses: actions/checkout@v4