ci(backend): Update GitHub backend QA workflow

Added caching and updated cargo-tarpaulin options in the GitHub
backend QA workflow. The cache setup addition uses
Swatinem/rust-cache at version 2, designed to improve workflow
efficiency.

.github/workflows/backend_qa.yml

@@ -26,6 +26,9 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Setup Cache
+        uses: Swatinem/rust-cache@v2
+
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
@@ -51,4 +54,4 @@ jobs:
           use-tool-cache: true
 
       - name: Run cargo-tarpaulin
-        run: cargo tarpaulin --implicit-test-threads
+        run: cargo tarpaulin --implicit-test-threads --skip-clean