Add support for Win isolate and isolate-server binaries to isolate.go
Add support for Win isolate and isolate-server binaries to isolate.go

Chromium builders no longer populate gs://chromium-luci which is why I
copied over the binaries to skia-public-binaries for now.
Eventually we will have to switch everything over to use
isolate/isolated from CIPD.

Also make isolate.go runnable from a Windows machine.

Bug: skia:8771
Change-Id: I6eaf1194908360e04e8969bf12f2df375e92b05b
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/197705
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot