Default to stable toolchain for metrics workflow

.github/workflows/metrics.yaml

@@ -19,6 +19,7 @@ jobs:
         run: |
           rustup update --no-self-update stable
           rustup component add rustfmt rust-src
+          rustup default stable
       - name: Cache cargo
         uses: actions/cache@v3
         with:
@@ -47,7 +48,6 @@ jobs:
             ~/.cargo/git/db/
           key: ${{ runner.os }}-cargo-${{ github.sha }}
 
-
       - name: Collect build metrics
         run: cargo xtask metrics build