cli-plugins: add build-time system plugin directory

[HeroCC]

Summary

Adds an optional system CLI plugin directory that downstream builds can set with the Go linker -X flag. This lets downstream packagers expose package-managed CLI plugins without requiring users to modify their Docker configuration.

If accepted, I plan on using this in homebrew's docker formula to avoid needing caveats (like this) for plugins.

Testing

• go test github.com/docker/cli/cli-plugins/manager
• go test -ldflags='-X github.com/docker/cli/cli-plugins/manager.additionalSystemPluginDir=/test/cli-plugins' github.com/docker/cli/cli-plugins/manager
• Built and installed a downstream package with the linker setting and confirmed its packaged Compose plugin is discovered.

$ docker info
Client: Docker Engine - Community
 Version:    HEAD-9eaf761
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.37.1
    Path:     /opt/homebrew/lib/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  5.5.1
    Path:     /opt/homebrew/lib/docker/cli-plugins/docker-compose

Release notes (optional)

Add support for configuring an additional system CLI plugin directory at build time.

A picture of a cute animal (not mandatory but encouraged)

Rex :)