Remove unneeded actions, update versions

Author: sourcefrog

.github/workflows/mutants.yaml

@@ -6,12 +6,8 @@ jobs:
   cargo-mutants:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
       - name: Install cargo-mutants
-        uses: baptiste0928/cargo-install@v2.1.0
+        uses: baptiste0928/cargo-install@v2
         with:
           crate: cargo-mutants
       - name: Run mutant tests