make.bat: return exit code to dashboard builder

Fixes #7806.

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/104690043

src/make.bat

@@ -117,3 +117,4 @@ goto end
 set GOBUILDFAIL=1
 
 :end
+if x%GOBUILDEXIT%==x1 exit %GOBUILDFAIL%