ci(ci): cache Rust artifacts in backend-tests and build jobs

Add Swatinem/rust-cache@v2.8.1 (SHA-pinned) after the Rust toolchain
install step in both macOS jobs that actually compile the Tauri crate:
backend-tests (cargo test via pnpm run health:repo:backend) and build
(pnpm tauri build). Skipped security-audit since cargo audit only
fetches the advisory DB — no compile benefit.

Configuration:
- workspaces: src-tauri — cache src-tauri/target not repo root
- shared-key: macos-rust — both jobs reuse registry/git caches
- cache-on-failure: true — keep cache when a single test regresses

Expected impact: ~2-3 min saved per CI run on each of the two macOS
jobs after first warm cache, by avoiding full src-tauri/target/
recompile from scratch.

Author: saagar210

.github/workflows/ci.yml

@@ -148,6 +148,13 @@ jobs:
         with:
           toolchain: stable
 
+      - name: Cache Rust build artifacts
+        uses: Swatinem/rust-cache@f13886b937689c021905a6b90929199931d60db1 # v2.8.1
+        with:
+          workspaces: src-tauri
+          shared-key: macos-rust
+          cache-on-failure: true
+
       - name: Run backend health lane
         run: pnpm run health:repo:backend
 
@@ -307,6 +314,13 @@ jobs:
         with:
           toolchain: stable
 
+      - name: Cache Rust build artifacts
+        uses: Swatinem/rust-cache@f13886b937689c021905a6b90929199931d60db1 # v2.8.1
+        with:
+          workspaces: src-tauri
+          shared-key: macos-rust
+          cache-on-failure: true
+
       - name: Install dependencies
         run: pnpm install --frozen-lockfile