Gruntjs: How do I build a dynamic destination path using grunt-contrib-copy?

Question

Not sure if this is possible, but I want to specify a variable to construct a destination path when using grunt-contrib-copy like so:

module.exports = function(grunt) {

    var $globals = {
        buildPath: 'default'
    };

    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),
        copy: {
            all: {
                files: [{
                    expand: true,
                    cwd: 'source/',
                    src: ['file1.js', 'file2.js'],
                    dest: 'builds/' + $globals.buildPath + '/'
                }]
            }
        }
    });

    grunt.loadNpmTasks('grunt-contrib-copy');

    grunt.registerTask('test', 'Build Test', function() {
        $globals.buildPath = 'test';
        grunt.task.run(['copy']);
    });

};

In the above example, the copy task uses the initial value set in $globals.buildPath, which is default and it copies the files into the /builds/default/ directory instead of what I want, which is /builds/test/

I know that I can create a new subtask to achieve the result that I want, but doing it dynamically would make my Gruntfile.js smaller and easier to manage.