add a missing cache step

.github/workflows/pr.yml

@@ -63,6 +63,7 @@ jobs:
       - run: rustup component add clippy
       - name: Install dependencies on Ubuntu
         run: sudo apt-get update && sudo apt-get install cmake pkg-config libgtk-3-dev
+      - uses: Swatinem/rust-cache@v1
       - uses: actions-rs/cargo@v1
         with:
           command: clippy