Cache build artifacts in CI

Our CI is somewhat slow, mostly owing to the long time it takes to build
the slew of dependencies we have.
To mitigate this problem, cache build artifacts so that they can be
reused between CI runs.

.github/workflows/test.yml

@@ -33,6 +33,9 @@ jobs:
     - uses: dtolnay/rust-toolchain@master
       with:
         toolchain: ${{ matrix.rust }}
+    - uses: Swatinem/rust-cache@v2
+      with:
+        key: ${{ matrix.rust }}-${{ matrix.profile }}
     - name: Build & test ${{ matrix.profile }}
       run: |
         cargo build --profile=${{ matrix.profile }} --bins
@@ -44,6 +47,7 @@ jobs:
       - uses: actions/checkout@v4
       - name: Install Nightly Rust
         uses: dtolnay/rust-toolchain@nightly
+      - uses: Swatinem/rust-cache@v2
       - run: cargo +nightly -Z minimal-versions update
       - name: Install minimum Rust
         uses: dtolnay/rust-toolchain@master
@@ -59,6 +63,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
       - run: cargo clippy --no-deps --all-targets --all-features --tests -- -A unknown_lints -A deprecated -D warnings
   rustfmt:
     name: Check code formatting