x/build: update OpenSSH version on Windows builders

[bcmills]

In the course of looking into #51602, I took a look at the Windows gomote configuration.

As best as I can tell (from looking at https://cs.opensource.google/go/x/build/+/master:env/windows/startup.ps1;l=112-124;drc=114c42eb0ee190263854336c7f1e8bc36d53c03f) the builder is running Win32-OpenSSH v0.0.18.0, which was tagged July 19, 2017.

When attempting to reproduce the test failure in #51602 on a local machine, I installed OpenSSH on my machine using the process described in https://docs.microsoft.com/en-us/windows-server/administration/openssh/openssh_install_firstuse#install-openssh-using-windows-settings, and sshd --version reports that Windows itself installs OpenSSH_for_Windows_8.1p1, LibreSSL 3.0.2. That appears to correspond to v8.1.0.0p1-Beta, tagged Dec. 18, 2019.

The most recent Win32-OpenSSH tag at the moment appears to be V8.6.0.0p1-Beta, tagged May 27, 2021.

In order to keep the Go project's builders in line with what Go users are likely to actually observe on their deployments — and to help keep up to date with any OpenSSH security fixes that may have been published in the interim — I suggest that we update the OpenSSH install on the Windows builders to at least v8.1.0.0p1-Beta.

(CC @golang/release, @golang/security)