fix: add fetch-busybox to ARM64 cross-compile and test jobs

Busybox binary is no longer committed to git (downloaded at build time
from fc-busybox release). Add fetch-busybox to:
- cross-compile job (with BUILD_ARCH=arm64 for correct binary)
- orchestrator ARM64 test setup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

.github/workflows/pr-tests-arm64.yml

@@ -31,6 +31,9 @@ jobs:
             echo "::endgroup::"
           done
 
+      - name: Fetch busybox for orchestrator embed
+        run: make -C packages/orchestrator fetch-busybox BUILD_ARCH=arm64
+
       - name: Build and vet orchestrator (CGO)
         run: |
           CGO_ENABLED=1 CC=aarch64-linux-gnu-gcc GOARCH=arm64 go build ./...
@@ -85,6 +88,9 @@ jobs:
 
       - name: Setup orchestrator tests
         run: |
+          # Download busybox for go:embed
+          make -C packages/orchestrator fetch-busybox
+
           # Enable unprivileged uffd (Ubuntu defaults to 0)
           echo 1 | sudo tee /proc/sys/vm/unprivileged_userfaultfd