Add Rust caching to GitHub actions

Author: mrbbot

.github/workflows/test.yml

@@ -17,6 +17,7 @@ jobs:
         with:
           profile: minimal
           toolchain: stable
+      - uses: Swatinem/rust-cache@v1
       - name: Use Node.js LTS
         uses: actions/setup-node@v2
         with: