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: