Enable GitHub cache for coverage build

Author: mgeisler

.github/workflows/coverage.yml

@@ -22,6 +22,8 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - uses: Swatinem/rust-cache@v2
+
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov