The _environ variable that is populated when linking libc on Windows does not support Unicode keys/values (or, at least, the encoding is not necessarily UTF-8). So, for Unicode support, _wenviron would need to be used instead. However, this means that the keys/values would be encoded as UTF-16, so they would need to be converted to UTF-8 before being returned by `os.getenv`. This would require allocation which is not part of the `os.getenv` API, so `os.getenv` is not implementable on Windows even when linking libc.

Closes https://github.com/ziglang/zig/issues/8456