scripts: skip downloads when workloads already exist

Add file-existence guards around firmware and OVMF download
calls in integration test scripts that were missing them.
Also guard prepare_linux() in test-util.sh so it returns
early when the kernel binary is already present.

This lets users pre-populate the workloads directory (e.g.
via CH_CUSTOM_KERNEL, CH_CUSTOM_FIRMWARE, CH_CUSTOM_OVMF)
and avoid redundant network fetches or source builds inside
the container.

Updated scripts:
- test-util.sh (prepare_linux early return)
- run_integration_tests_aarch64.sh
- run_integration_tests_vfio.sh
- run_integration_tests_windows_x86_64.sh
- run_integration_tests_windows_aarch64.sh

Assisted-by: GitHub Copilot:Claude-Opus-4.6

Signed-off-by: Muminul Islam <muislam@microsoft.com>

scripts/run_integration_tests_windows_x86_64.sh

@@ -16,9 +16,12 @@ fi
 WIN_IMAGE_FILE="/root/workloads/windows-server-2025-amd64-1.raw"
 
 WORKLOADS_DIR="/root/workloads"
+OVMF_FW="$WORKLOADS_DIR/CLOUDHV.fd"
 
 # Download amd64 ovmf
-download_amd64_ovmf
+if [ ! -f "$OVMF_FW" ]; then
+    download_amd64_ovmf
+fi
 
 CFLAGS=""
 if [[ "${BUILD_TARGET}" == "x86_64-unknown-linux-musl" ]]; then