ci: switch to Swatinem/rust-cache to fix macOS build (#90)

Caching ~/.cargo/bin/ via actions/cache@v4 was poisoning the cargo
shim on macOS runners — restored cargo resolved to rustup-init and
failed with "unexpected argument 'build' found". Swatinem/rust-cache
skips that directory and handles target/ pruning + key derivation.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -20,17 +20,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Cache cargo
-        uses: actions/cache@v4
-        with:
-          path: |
-            ~/.cargo/bin/
-            ~/.cargo/registry/index/
-            ~/.cargo/registry/cache/
-            ~/.cargo/git/db/
-            target/
-          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-
+        uses: Swatinem/rust-cache@v2
 
       - name: Run tests
         run: cargo test --verbose
@@ -61,17 +51,9 @@ jobs:
           components: clippy
 
       - name: Cache cargo
-        uses: actions/cache@v4
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/bin/
-            ~/.cargo/registry/index/
-            ~/.cargo/registry/cache/
-            ~/.cargo/git/db/
-            target/
-          key: ${{ runner.os }}-cargo-clippy-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-clippy-
+          key: clippy
 
       - name: Run clippy
         run: cargo clippy -- -D warnings
@@ -89,17 +71,9 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Cache cargo
-        uses: actions/cache@v4
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/bin/
-            ~/.cargo/registry/index/
-            ~/.cargo/registry/cache/
-            ~/.cargo/git/db/
-            target/
-          key: ${{ runner.os }}-cargo-build-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-build-
+          key: build
 
       - name: Build
         run: cargo build --release --verbose