Better support for Powershell on Kubernetes

What does this MR do?

Powershell Core supported for Kubernetes was added some time ago, because it worked for Linux.

With the introduction of Windows on Kubernetes, we also need to support Powershell, which has some subtle differences.

Why was this MR needed?

• Powershell needs script file extensions, so we needed to add them.
• Powershell defaults to utf32 output, which means our log processing for a JSON line would fail with an unmarshaling error. The output is now utf8, so the build can terminate as expected.

What's the best way to test this MR?

• Run a any linux job, to ensure that the extension script change continues to work.
• Run a windows job with shell = "powershell". It should no longer hang indefinitely.

For the Linux job, our integration tests should already be covering this case multiple times, so it's likely only required to be tested manually on Windows.

What are the relevant issue numbers?

Closes #28153 (closed)