Bug #20958: fix ENV.keys encoding on windows

• Status: Open
• Assignee: -
• Target version: -
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[YO4 (Yoshinao Muramatsu)]

github PR#12368
ENV.keys elements contain UTF-8 byte sequences with locale encoding on Windows.

> ruby -v -e "p ENV.keys.last; p ENV.keys.last.encoding; p ENV.keys.last.dup.force_encoding('utf-8')"
ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x64-mingw-ucrt]
"\x{E383}\x{86E3}\x{82B9}\x{E383}\x88"
#<Encoding:Windows-31J>
"テスト"

This patch changes the encoding of ENV.keys to utf-8 on Windows.