Update GitHub actions in basic CI example (#342)

This updates the `checkout` action version to the latest (v4), since v2
produces warnings about deprecated Node.js version.

Also, the use of
[`actions-rs/toolchain`](https://github.com/actions-rs/toolchain) is
removed as the action is no longer maintained.

examples/workflows/basic.yml

@@ -27,10 +27,7 @@ jobs:
   cargo-mutants:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
+      - uses: actions/checkout@v4
       - uses: taiki-e/install-action@v2
         with:
           tool: cargo-mutants