ci(e2e): switch FUSE Mount build to Azure Ubuntu mirror, persist buildx cache

archive.ubuntu.com from GitHub-hosted runners has been Ign:/retrying for
~60s per package, eating the Start SeaweedFS step's 10-min budget before
apt-get install finishes. The host already uses azure.archive.ubuntu.com;
do the same inside Dockerfile.e2e and drop the Retries=5 amplifier.

Also rotate /tmp/.buildx-cache-new over /tmp/.buildx-cache so the apt
layer actually survives across runs, and bump the step to 15 minutes
as a safety margin.

.github/workflows/e2e.yml

@@ -57,12 +57,12 @@ jobs:
         echo "FUSE device: $(ls -la /dev/fuse 2>&1 || echo '/dev/fuse not found')"
 
     - name: Start SeaweedFS
-      timeout-minutes: 10
+      timeout-minutes: 15
       run: |
         # Enable Docker buildkit for better caching
         export DOCKER_BUILDKIT=1
         export COMPOSE_DOCKER_CLI_BUILD=1
-        
+
         # Build with retry logic
         for i in {1..3}; do
           echo "Build attempt $i/3"
@@ -77,10 +77,18 @@ jobs:
             sleep 30
           fi
         done
-        
+
         # Start services with wait
         docker compose -f ./compose/e2e-mount.yml up --wait
 
+    - name: Rotate buildx cache
+      if: always()
+      run: |
+        # Without this, --cache-to writes to .buildx-cache-new but actions/cache only
+        # uploads .buildx-cache, so layers (notably the slow apt RUN) never persist.
+        rm -rf /tmp/.buildx-cache
+        if [ -d /tmp/.buildx-cache-new ]; then mv /tmp/.buildx-cache-new /tmp/.buildx-cache; fi
+
     - name: Run FIO 4k
       timeout-minutes: 15
       run: |