Based of @samalba's proposal for CI #1230

• This adds a -t / --target (placeholder name) flag to dagger up
• Only that part of the DAG will execute
• All the necessary dependencies will be executed as well

Example:

engine.#Plan & {
	actions: {
		image: engine.#Pull & {
			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
		}

		exec: engine.#Exec & {
			input: image.output
			args: ["sh", "-c", "echo -n hello world > /output.txt"]
		}
	}
}

• dagger up -t actions.image will only run actions.image
• dagger up -t actions.exec will run actions.image and then actions.exec (because actions.image is necessary to run actions.exec)