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You can do that on the GitRepo

repo := GitRepo{
	URL: "<https://github.com/your/repo.git>",
	// this call back will get executed post-clone to allow for additional program setup
	Setup: func(ctx context.Context, workspace Workspace) error {
		cmd := exec.Command("npm", "install")
		cmd.Dir = workspace.WorkDir()
		return cmd.Run()
	},
}

Marvellous!