Use checkout v4 in gha

Author: dividehex

.github/workflows/rust.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Run fmt
       run: cargo fmt --all -- --check
     - name: Clippy