Set the toolchain version

.github/workflows/mutants.yaml

@@ -3,11 +3,13 @@ name: cargo-mutants
 on: [pull_request, push]
 
 jobs:
-  mutation:
+  cargo-mutants:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
       - name: Install cargo-mutants
         run: cargo install cargo-mutants
       - name: Run mutant tests