main.star: pass machine_type in expected_dimensions for Windows builders

This will allow us to distinguish which builders get n4d machines and
which get n1 machines.

For golang/go#82017

Change-Id: I44a2a11ea76fdf7dfa428cd0824e543d6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/build/+/850165
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>