CI: Update to actions/checkout@v3

No functional change, but we should keep this current.
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index a9e86b6..e413b20 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -23,7 +23,7 @@
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Generate Build Matrix
         id: set-matrix
         run: python3 .github/matrix.py "${{ github.event_name }}"
@@ -44,7 +44,7 @@
       ASAN_OPTIONS: log_path=asan.log
       OT_CPP_VERSION: 1.6.0
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
         fetch-depth: 100
 #