Remove duplicate regexp phrase: ^\A

Addresses @mastahyeti's comment in #3077: https://github.com/jekyll/jekyll/pull/3077#discussion_r20077150

No clue how the tests passed with that invalid regexp. It's frustrating not being able to CI this on Windows.
So, if you're running this in safe mode with Windows, an attacker could write to the correct path, but on any drive, right?