builder: set $GOBUILDEXIT for Windows

Actually %GOBUILDEXIT% I suppose.

R=golang-dev
CC=golang-dev
https://golang.org/cl/5651058

misc/dashboard/builder/main.go

@@ -478,6 +478,7 @@ func (b *Builder) envv() []string {
 		"GOOS=" + b.goos,
 		"GOARCH=" + b.goarch,
 		"GOROOT_FINAL=/usr/local/go",
+		"GOBUILDEXIT=1", // On Windows, exit all.bat with completion status.
 	}
 	for _, k := range extraEnv {
 		s, err := os.Getenverror(k)

src/all.bat

@@ -16,4 +16,4 @@ if %GOBUILDFAIL%==1 goto end
 ..\bin\tool\dist banner
 
 :end
-exit %GOBUILDFAIL%
+if x%GOBUILDEXIT%==x1 exit %GOBUILDFAIL%