Add integration tests for Linux providers

## Description
Add integration tests for Linux-only providers: nsjail, gVisor, and Firecracker.

These providers require Linux with specific dependencies and cannot be tested on macOS/Windows.

## Requirements
- [ ] Set up GitHub Actions workflow for Linux testing
- [ ] Integration tests for nsjail provider
- [ ] Integration tests for gVisor provider (requires runsc)
- [ ] Integration tests for Firecracker provider (requires KVM)
- [ ] Test matrix for different Linux distributions

## Dependencies
- nsjail: https://github.com/google/nsjail
- gVisor (runsc): https://gvisor.dev/docs/user_guide/install/
- Firecracker: https://github.com/firecracker-microvm/firecracker