x/build: add a Windows Server, version 1803 (~"Windows 10") builder

[bradfitz]

GCE supports "Windows Server, version 1803":

https://cloud.google.com/compute/docs/instances/windows/

We should run such a builder.

See:
https://go-review.googlesource.com/c/go/+/131976
https://docs.microsoft.com/en-us/windows-server/get-started/get-started-with-1803

It would've prevented #25722.

/cc @johnsonj (who noted these things on CL 131976), @dmitshur