fix(windows): fix encoding issues while verifying signatures

[orzFly]

Our code signing certificate contains Chinese company name in the CN field. Encoding issues may be complex on Windows so I just encode the powershell output as base64-ed UTF-8 bytes.

[hezhuojie]

Holyshit.
It convert CN from gbk encoded to utf-8 in checkForUpdate.
But in publish stage, windowsCodeSign automatically extracts publisher name of gbk encoded from code signing certificate in. The gbk encoded publish name is wrote to app-update.yml and is used for checkForUpdate.
So AppUpdater must fail to verify signature due to compare utf-8 encoded publish name and gbk encoded name.

[develar]

@hezhuojie Thanks for finding, PR welcome :)