Fix MMLU answer extraction regex for repeated "Answer: LETTER" pattern

[lucasresck]

Description

This pull request addresses issue https://github.com/openai/simple-evals/issues/33 by fixing the regular expression used to extract answers from model outputs with MMLU.

Solution

The existing regex fails to handle cases where the "Answer: LETTER" pattern appears multiple times. This is resolved by:

• Using re.findall: Instead of re.search, re.findall is used to find all occurrences of the answer pattern.
• Selecting the last match: The last match from the re.findall results is taken as the correct answer.
• Allowing overlapping matches: The regex pattern is adjusted to allow overlapping matches, using a capturing group inside a lookahead.