Add ARM (arm64/aarch64) architecture support

[jBouyoud]

Context

Currently, copa-action only supports the amd64 architecture. With the growing adoption of ARM-based runners (GitHub Actions arm64 runners, AWS Graviton, Apple Silicon self-hosted runners), users running workflows on ARM infrastructure cannot use this action.

Need

As a user running GitHub Actions on ARM-based runners, I want copa-action to support the arm64 architecture so that I can patch container images without being restricted to amd64 runners.

Expected changes

• Download and install the appropriate Copa binary based on the runner's architecture (amd64 or arm64)
• Ensure buildkit / trivy components used by the action also support ARM
• Update any architecture-hardcoded references to dynamically detect RUNNER_ARCH or uname -m

Additional context

• GitHub now offers arm64 hosted runners
• The Copa CLI itself already publishes linux/arm64 binaries in its releases