Pin to specific versions of GitHub Actions Runners (#349)

Author: philrz

.github/workflows/ci.yaml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-latest, ubuntu-latest, windows-latest]
+        os: [macos-12, ubuntu-20.04, windows-2022]
     steps:
       - uses: actions/checkout@v4
         with: