CI,GH: Switch to v3 of `checkout` action

.github/workflows/rust.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Setup
         run: |
           sudo mkdir -p /run/user/$UID |