ci: cache Scylla download across CI matrix jobs

Add an actions/cache step for ~/.ccm/repository keyed on the Scylla
version and runner OS. On cache hit the 'Download Scylla' step
becomes a near-instant no-op. On miss (or version bump) CCM
re-downloads as before, so there is no regression risk.

Author: mykaul

.github/workflows/integration-tests.yml

@@ -77,6 +77,12 @@ jobs:
     - name: Build driver
       run: uv sync
 
+    - name: Cache Scylla download
+      uses: actions/cache@v4
+      with:
+        path: ~/.ccm/repository
+        key: scylla-${{ env.SCYLLA_VERSION }}-${{ runner.os }}
+
     # This is to get honest accounting of test time vs download time vs build time.
     # Not strictly necessary for running tests.
     - name: Download Scylla