Don't use upx with Windows binaries

ref: upx/upx#154

libbuild.py

@@ -210,7 +210,7 @@ def go_build(name, goos, goarch, main, compress=False, upx=False):
         )
     die(call(cmd, cwd=REPO_ROOT))
 
-    if upx and (goarch in ['amd64', '386']):
+    if upx and (goos in ['linux', 'darwin']) and (goarch in ['amd64', '386']):
         cmd = "upx --brute {name}-{goos}-{goarch}{ext}".format(
                 name=name,
                 goos=goos,