Cache build artifacts in build-minimum CI job

With this change we use the Swatinem/rust-cache GitHub action to cache
Rust build artifacts for the build-minimum CI job, which currently takes
the most time to complete. In so doing we speed up CI runs.

.github/workflows/test.yml

@@ -63,6 +63,7 @@ jobs:
           # bumping version.
           toolchain: 1.56
           default: true
+      - uses: Swatinem/rust-cache@v2
       - name: Build
         run: cargo build --locked --all-features
   rustfmt: