Follow up to #2603

Prior to that PR, when we first launched, we'd set powershell core to the default profile if it was installed, instead of Powershell.

With the switch to dynamic profiles, we stopped doing that. Windows Powershell is now always the default.

We should restore that behavior. The check should be simple. Modifying the user's defaultProfile in place might be a bit trickier. Adding tests also takes time, and we want to get #2515 out the door sooner than later.