utils: Convert Windows args to utf-8 string

Summary: This is a backport of Core PR13883

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4174