Fix CI disk image build to avoid GTK error

- Change from 'cargo run' to 'cargo build' to avoid running QEMU during build
- Add step to copy disk image to expected location for QEMU run
- This prevents GTK initialization failed error in CI environment

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ryanbreen

.github/workflows/isolation-syscall.yml

@@ -37,7 +37,11 @@ jobs:
       run: cargo build --release --features testing
 
     - name: Build disk image
-      run: cargo run --release --bin qemu-uefi --features testing
+      run: |
+        cargo build --release --bin qemu-uefi --features testing
+        # Copy disk image to expected location
+        mkdir -p target/x86_64-unknown-none/release/
+        cp $(find target/release/build -name "breenix-uefi.img" -type f | head -1) target/x86_64-unknown-none/release/breenix-uefi.img
     
     - name: Run kernel and capture logs
       run: |