add build artifact caching

Author: sunshowers

.github/workflows/ci.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           toolchain: stable
           components: rustfmt, clippy
+      - uses: Swatinem/rust-cache@c5ed9ba6b7e1bb8aff90d43acd2f0af4990fa57c
       - name: Lint (rustfmt)
         uses: actions-rs/cargo@v1
         with:
@@ -51,6 +52,7 @@ jobs:
         with:
           toolchain: stable
           override: true
+      - uses: Swatinem/rust-cache@c5ed9ba6b7e1bb8aff90d43acd2f0af4990fa57c
       - name: Build
         uses: actions-rs/cargo@v1
         with: