Enable cargo cache for CI coverage job

No need to compile the dependencies from scratch every time.

.github/workflows/pull-request.yaml

@@ -107,6 +107,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - uses: Swatinem/rust-cache@v1
       - run: rustup component add llvm-tools-preview
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov