fix: handle paddlepaddle-gpu CUDA import at build time gracefully

paddlepaddle-gpu requires libcuda.so.1 at import time, but no CUDA
driver exists during Docker build. The download script now catches
this ImportError and skips PaddleOCR model pre-download on amd64.
Models will download on first use at runtime when the CUDA driver
is available via nvidia-container-toolkit.

On arm64 (CPU paddlepaddle), models are still pre-downloaded at build
time as before.

Also reverted CI to amd64-only Docker build test for speed. Multi-arch
build is tested on release via the release workflow.

.github/workflows/ci.yml

@@ -87,14 +87,11 @@ jobs:
 
       - uses: docker/setup-buildx-action@v3
 
-      - uses: docker/setup-qemu-action@v3
-
       - uses: docker/build-push-action@v6
         with:
           context: .
           file: docker/Dockerfile
           push: false
-          platforms: linux/amd64,linux/arm64
           tags: stirling-image:ci
           cache-from: type=gha,scope=unified
           cache-to: type=gha,mode=max,scope=unified