env_setup: package protoc-gen-go for Windows

This change adds Windows to the supported systems for protoc-gen-go in
pigweed.ensure.

Change-Id: Id35193f01694c78520481105526783933e398156
diff --git a/env_setup/cipd/pigweed.ensure b/env_setup/cipd/pigweed.ensure
index 2adbb3a..fecba25 100644
--- a/env_setup/cipd/pigweed.ensure
+++ b/env_setup/cipd/pigweed.ensure
@@ -58,7 +58,7 @@
 infra/go/${platform} version:1.13.5
 
 # Go protobuf plugin
-pigweed/third_party/protoc-gen-go/${os=linux,mac}-${arch} version:1.3.2
+pigweed/third_party/protoc-gen-go/${platform} version:1.3.2
 
 # OpenOCD
 # TODO(pwbug/66) Put openocd in cipd for Windows.