Change in handling of IdentityAgent on Windows OpenSSH 8.9

Submitting this here in case it helps anyone else. The short version is:

```
IdentityAgent \\.\pipe\krd-agent
``` 
in my ssh config stopped working; either removing that line and setting $env:SSH_AUTH_SOCK or changing it to

```
IdentityAgent //./pipe/krd-agent
```

fixed it for me. See https://github.com/PowerShell/Win32-OpenSSH/issues/1949.
