GitHub Actions: Update actions/checkout to v4

Fixes warnings regarding deprecated node12

.github/workflows/test.yml

@@ -31,7 +31,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       # System set-up
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: ilammy/msvc-dev-cmd@v1
       - uses: seanmiddleditch/gha-setup-ninja@master
       - uses: jwlawson/actions-setup-cmake@v2