build: use setlocal in run.bat

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5672061

src/all.bat

@@ -13,7 +13,7 @@ goto end
 
 call make.bat --no-banner --no-local
 if %GOBUILDFAIL%==1 goto end
-call run.bat --no-rebuild
+call run.bat --no-rebuild --no-local
 if %GOBUILDFAIL%==1 goto end
 go tool dist banner
 

src/run.bat

@@ -3,6 +3,13 @@
 :: license that can be found in the LICENSE file.
 @echo off
 
+:: Keep environment variables within this script
+:: unless invoked with --no-local.
+if x%1==x--no-local goto nolocal
+if x%2==x--no-local goto nolocal
+setlocal
+:nolocal
+
 set GOBUILDFAIL=0
 
 rem TODO avoid rebuild if possible