Fix exec call to remove cache folder

[victorb]

https://github.com/VictorBjelkholm/trymodule/blob/2030806d790213d426456959fdc7a69773a4c6f7/index.js#L59

Oh noes. Please don't. Never do string concat for commands, use execFile instead. Never pass environment variables directly to rm without sanitation, better yet, never spawn rm. Use a JS tools, like rimraf or even better del.

From: https://github.com/sindresorhus/awesome-nodejs/pull/477#event-606502245