fix: start-runner.ps1 set username

[sdarwin]

This is to facilitate discussion of issue #4712

Usually the username on windows comes from runner_run_as: Administrator. Recently that has broken and it's always System.

In start-runner.ps1 move everything back inside a "Register-ScheduledTask" so the username can be set. This fixes the issue.

However, start-runner.ps1 also included a relatively new section:

Write-Host "Terminating instance"
aws ec2 terminate-instances --instance-ids "$InstanceId" --region "$Region"

By removing it, instances fail to terminate. How did this work before? ( in 2023) Why is it necessary to terminate instance from start-runner.ps1 instead of how it used to work before?

By removing terminate-instances, the instances continue to run, which is obviously not the desired result.

Where should terminate-instances be placed, or can the earlier solution handle that task.

[sdarwin]

@npalm this PR has been tested and is ready to review.

Closes #4712