Powershell: fix unwanted progress streams leaking to output

What does this MR do?

!3707 (merged) forced all commands to be encoded in base64 to avoid UTF-8 encoding issues.

However, this caused unwanted progress output to be leaked with each command due to an upstream bug: https://github.com/PowerShell/PowerShell/issues/5912.

This commit reverts he previous behavior of using unencoded commands for powershell, but leaves in the encoding for pwsh.

Relates to #29462 (closed)